doi.org/10.26434/chemrxiv.12098142.v1

A Pseudo Five-Fold Symmetrical Ligand Drives Geometric Frustration inPorous Metal-Organic and Hydrogen Bonded FrameworksFrederik Haase, Gavin Craig, Mickaele Bonneau, kunihisa sugimoto, Shuhei Furukawa

Submitted date: 08/04/2020 • Posted date: 09/04/2020Licence: CC BY-NC 4.0Citation information: Haase, Frederik; Craig, Gavin; Bonneau, Mickaele; sugimoto, kunihisa; Furukawa,Shuhei (2020): A Pseudo Five-Fold Symmetrical Ligand Drives Geometric Frustration in PorousMetal-Organic and Hydrogen Bonded Frameworks. ChemRxiv. Preprint.https://doi.org/10.26434/chemrxiv.12098142.v1

Reticular framework materials thrive on designability, but unexpected reaction outcomes are crucial inexploring new structures and functionalities. By combining “incompatible” building blocks, we employedgeometric frustration in reticular materials leading to emergent structural features. The combination of apseudo C5 symmetrical organic building unit based on a pyrrole core, with a C4 symmetrical copperpaddlewheel synthon led to three distinct frameworks by tuning the synthetic conditions. The frameworksshow structural features typical for geometric frustration: self-limiting assembly, internally stressed equilibriumstructures and topological defects in the equilibrium structure, which manifested in the formation of ahydrogen bonded framework, distorted and broken secondary building units and dangling functional groups,respectively. The influence of geometric frustration on the CO2 sorption behavior and the discovery of a newsecondary building unit shows geometric frustration can serve as a strategy to obtain highly complex porousframeworks.

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A pseudo five-fold symmetrical ligand drives geometric frustration in porous metal-organic and hydrogen bonded frameworks Frederik Haase,*,a Gavin A. Craig,a Mickaële Bonneau,a Kunihisa Sugimoto,a,b 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

Supporting Information Placeholder

ABSTRACT: Reticular framework materials thrive on designability, but unexpected reaction outcomes are crucial in exploring new structures and function-alities. By combining “incompatible” building blocks, we employed geometric frustration in reticular ma-terials leading to emergent structural features. The combination of a pseudo C5 symmetrical organic building unit based on a pyrrole core, with a C4 sym-metrical copper paddlewheel synthon led to three distinct frameworks by tuning the synthetic condi-tions. The frameworks show structural features typ-ical for geometric frustration: self-limiting assembly, internally stressed equilibrium structures and topo-logical 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 CO2 sorption behavior and the discovery of a new second-ary building unit shows geometric frustration can serve as a strategy to obtain highly complex porous frameworks.

Reticular framework materials are a class of materi-als that are constructed by the selection or design of building blocks and linkage groups such that a de-sired net is formed.1 This approach is successfully applied for the introduction of functionality on the organic linkers and on known metal based secondary building units (SBU),2 while structural complexity can be increased by the rational construction of new organic nodes and new nets3 or by the synthesis of controlled multivariate materials.4,5 Recently, design strategies have emerged 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 symmetry, new nets and open metal sites.1 This can be achieved

by linker clip-off,6 dissolution and introduction of defects7–10 or the installation of size and shape mis-matched linkers.1 These approaches are all based on the knowledge of a well-defined parent lattice. Our strategy here is to use mismatched symmetry in or-der to introduce geometric frustration, where the mismatch is large enough to prohibit the formation of predetermined lattices or nets. Geometric frustra-tion arises when local geometric constraints hinder the energetic minimization of all of the interactions at the same time.11 This is a sought after phenomenon for spin ices in magnetic materials12 and their appli-cation in data storage,13 but the use of competing in-teractions to obtain complex systems is a widespread phenomenon.12 We therefore wanted to induce geo-metric frustration, in order to explore new emergent structures and features12,14 without preconception of the reaction outcome.

In this work, we introduced geometric frustration through a linker with preudo-C5 symmetry, as the symmetry cannot be propagated to a classical peri-odic lattice based-on translation, while periodic lat-tices based on rotation and translation such as in a quasicrystal could support this symmetry.11,15 Sev-eral strategies exist for the introduction of C5 sym-metry in organic building blocks,16–19 as these are used for the synthesis of large cage coordination compounds.18,20,21 As introducing C5-symmetry can be synthetically challenging,19,22 instead we designed a pseudo-C5 symmetrical ligand based on a pyrrole core (see method section, Figure S1). The pseudo-C5 symmetric linker containing pendant carboxylic acid groups, 1,2,3,4,5-penta (4-(benzoic acid))pyrrole (pLH5; Figure 1) was reacted with copper(II) ions as a synthon for the C4 -symmetric paddlewheel motif. While metal-organic frameworks based on paddle-wheel have been reported with planar 2-c,23 3-c,24 4-c25 and 6-c26 nodes before, a search of MOF data-bases27 revealed no nets with alternating 5-c and 4-c

nodes. The combination of C4 and C5 local symme-tries was, therefore, expected to be difficult to rec-oncile while maintaining the symmetry of the build-ing blocks.

Figure 1. Schematic illustration of the synthesis of materials from the pentagonal linker pLH5.

Using pLH5 for the synthesis of frameworks, three materials were crystallized: a hydrogen bonded framework (HOF), based on a molecular copper(II) complex containing four [pLH4]- units (Cu2(pLH4)4; pentaHOF-1) and two different metal-organic frame-works (MOF) consisting of [pLH]4- units (Cu2(pLH); pentaMOF-2 and pentaMOF-3). The 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 so-lution with a solution of pLH5 in ethanol at room temperature leads to the formation of a metal con-taining HOF (pentaHOF-1). This material consists of a molecular copper complex Cu2(pLH4)4 that is com-posed of a single copper paddlewheel surrounded by four [pLH4]- moieties in a propeller shape (Figure 2a). The pentaHOF-1 crystallizes in the tetragonal space group P4/nnc (Table S1) in which the Cu2(pLH4)4 molecules 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 in-teractions where one Cu2(pLH4)4 molecule interacts with 10 of its neighbors (8 from the body centered arrangement and 2 from interaction with the mole-cules above and below the Cu2(pLH4)4 molecules; Fig-ure S2, Figure S3) through its 16 carboxylic acids. The structure contains only a single hydrogen bond-ing motif where four carboxylate groups bond to each other by forming three hydrogen bonds (Figure S4). Two crystallographically independent oxygen-oxygen distances (dO---O) were observed, which can

be assigned to moderately strong hydrogen bonds28 with dO---O of 2.74(18) Å and dO---O of 2.65(15) Å. The pentaHOF-1 structure contains two types of 1D chan-nels parallel to the c-axis. The first type is purely in-termolecular, created between stacks of Cu2(pLH4)4 molecules with pore apertures of 11.3×11.3 Å2 (indi-cated as A in Figure S5). The second type of void is created by connecting the inter-molecular voids within one stack of molecules by small intramolecu-lar apertures (4.6×3.8 Å2) (indicated as B in Figure S5).

Figure 2. Detail illustrations of the crystal structures of (a,b) pentaHOF-1, (c,d) pentaMOF-2 and (e,f) pen-taMOF-3, which emphasize the connectivity of the structure.

Changing the solvent system led to the formation of a MOF with the composition Cu2(pLH). pentaMOF–2 single crystals were obtained with monoclinic crys-tal 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 a fully

3D metal-organic network containing [pLH]4- units and copper paddlewheels. Two out of four carbox-ylate groups create a square lattice layer with regu-lar copper paddlewheels (Figure 2c), while the re-maining two carboxylate groups twist out of plane and connect these layers with a distorted copper paddlewheel leading to a sqc17629 (lvt) topology (Figure 2d). The fith, potential carboxylate group re-mains protonated and is therefore not involved in binding to copper. The carboxylic acid group pro-trudes 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 bond-ing 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).

By increasing the reaction temperature up to 110°C and using an n-butanol/water mixture, we obtained single crystals of pentaMOF-3 in an orthorhombic Pna21 space group (Table S1) with the same compo-sition Cu2(pLH) as pentaMOF-2, but with a different connectivity. The structure is composed of [pLH]4- 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 pro-tonated and able to form a hydrogen bond with a sec-ond linker with an average dO---O of 2.75(13) Å (Figure S4). The copper binds the linker with two distinct secondary building units (SBU) (Figure 2f). 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 coordinates to one copper in an equatorial fashion and through pseudo-coordination to the axial position of a neighboring copper pair, thus leading to a one-dimensional SBU. The copper pairs are bridged by three axial carboxylate anions with a dCu---Cu 2.98 Å, which is considerably larger than in the reg-ular paddlewheels present in the structure with dCu---Cu = 2.64 Å. The inter-pair distance, bridged by an axial-equatorial carboxylate binding motif is even larger with dCu---Cu = 4.67 Å. The pseudo-coordina-tion of the rod SBU and the hydrogen bonding con-nect the individual layers with each other, leading to a 3D net based on the sqc5429 topology (Figure S8). The structure contains small undulating 1D channels with an aperture of 5.5×3.6Å2 (Figure S9).

The single crystal structures of the materials pre-sented here showed promising solvent accessible voids, and we therefore investigated gas sorption properties. Only the pentaHOF-1 showed uptake of N2 at 77 K with a type-I isotherm, with a BET surface

area of 849 m2g-1 (Figure 3a), which confirms its per-manent porosity.

Figure 3. Sorption measurement of pentaHOF-1, pen-taMOF-2 and pentaMOF-3 for (a) N2 at 77 K and (b) CO2 at 195 K.

In contrast, all three materials showed CO2 adsorp-tion at 195K. The isotherm of pentaHOF-1 exhibits a steep rise at low partial pressures that includes a step at around 0.055 P/P0 (Figure 3b, inset). Such a stepped increase could be attributed to a dynamic feature of the framework material30 or a preferential adsorption site.31 The single crystal structure of pen-taHOF-1 shows copper paddlewheels situated di-rectly above each other with a Cu---Cu separation of 7.37 Å, which conforms well with a previously de-scribed “optimum” distance for the binding of CO2.31 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 CO2 isotherms of pentaMOF-2 and -3 described a type-I shape with a total uptake significantly smaller than pentaHOF-1. The differences in total CO2 uptake between the three structures are in stark contrast to the available metal binding sites per gram of mate-rial. 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, 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 CO2, which is small compared to the total uptake of 297.5 ml/g (~5% of the total uptake). This result is expected considering the pore size of the materials, with pentaMOF-2 and -3 possessing only small pores, while pentaHOF-1 has a much larger channel with a ~1.1 nm diameter. On the other hand, it is surprising, that a hydrogen bonded struc-ture of an “intermediate” possesses larger porosity than the corresponding MOFs. Normally, the for-mation 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.

Grason11 offers a theoretical context for interpret-ing the effects of geometric frustration by defining anomalous structural properties such as “internally stressed equilibrium structures, self-limiting assem-bly, and topological defects in the equilibrium as-sembly structures”.11 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 pen-taHOF-1 this is observed in an extreme fashion as the molecule Cu2(pLH4)4 consists of only the first sphere of coordination, as a single copper paddlewheel is surrounded by four pL molecules that are not con-nected to further metal SBU. This is surprising, since under the applied synthetic conditions, a full reac-tion could be expected, as it is possible to synthesize copper paddlewheel MOFs such as, for example, HKUST-1 at room temperature.24 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 coordina-tion environments, as the angles and distances of the carboxylic acid groups cannot be rearranged to ac-commodate the energetic minimum for an un-strained copper paddlewheel. In pentaHOF-1, the copper paddlewheel (Figure 4) and the pL molecules are unstrained as a result of the formation of a dis-crete molecule.

However, the structures of pentaMOF-2 and pen-taMOF-3 show a copper paddlewheel that is more distorted than in any previously reported structure in the Cambridge structural database32 (CSD) (Figure 4). In pentaMOF-2 one paddlewheel remains largely unaffected by strain, while the other is markedly dis-torted. The pentaMOF-3 structure shows one paddle-wheel that is distorted more strongly than in

pentaMOF-2, and additionally the second SBU is strained so strongly that the paddlewheel motif breaks down and a rod SBU is formed instead. The copper SBU is experiencing strain in the equatorial plane and also in the axial direction (Figure 2f, Fig-ure S10) as a consequence of the partially overlap-ping linkers (Figure 2e) within one layer of pen-taMOF-3. The equatorial strain can be resolved by an in-plane change of the bite angle, while the axial strain is released by altering the binding motif from a paddlewheel to a rod SBU. This effect can be quan-tified by calculating the out-of-plane distortion of the [pLH]4- carboxyphenyl groups in relation to the central pyrrole core. The groups connected to the rod SBU show a significantly reduced out-of-plane bend-ing compared to those connected to the copper pad-dlewheels (Table S2).

Figure 4. Distortion of the copper paddlewheel structure of pentaHOF-1, pentaMOF-2 and pen-taMOF-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 equilib-rium structure lead to coordination nets based on a 4-c node for the nominally 5-c pLH5 linker (Figure 2e). This can be understood as a result of the diffi-culty translating the C5 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 re-ticulation, which is then resolved by dangling car-boxylic acids.

Applying this analysis to other reported MOFs, re-veals geometric frustration leading to anomalous structures such as in the formation of a hydrogen bonded material consisting of metal organic polyhe-dra instead of a MOF33 and the formation distorted or untypical metal SBU due to incompatibilities be-tween the linker and the SBU (see also supplemental discussion).34

Figure 5. Qualitative assessment of the competing forces that lead to frustration (details can be found in Figure S11). Values further from the center indi-cate a stronger penalty.

The hallmark of geometric frustration is the ab-sence of a simple ground state, which leads to struc-tural solutions that show compromises of competing forces. In the structures described here, these forces are derived from metal-to-ligand bonding, metal co-ordination environment, hydrogen bonding, hydro-phobic interactions, porosity and geometric con-straints of the linker. Figure 5 exemplifies the diver-gent pathways the individual structures take to re-lease frustration. Each structure occupies a different area in this qualitative frustration diagram, showing the structural trade-offs arising from frustration. The most prominent driving force in MOF formation is the formation of the metal-ligand bond, however the pL geometry has such a strong influence on the system that protonated carboxylic acids remain in

the resulting structures. In pentaHOF-1, only one of five carboxylate groups bind to copper. Further car-boxylate groups can only bind to copper, by a signif-icant distortion of the copper paddlewheel motif as seen in pentaMOF-2 and -3. The largest difference between the MOFs pentaMOF-2 and -3 are observed in the trade-off between the distortion of the copper paddlewheel and the extent of hydrophobic interac-tions between the linker backbones. The pentaMOF-2 has completely isolated linkers, which allow it to reduce the distortion of the paddlewheels, while the pentaMOF-3 has extensive linker-to-linker contacts, but has dramatically increased SBU distortion.

These results indicate that the comparatively weak interactions between linkers play a significant role in determining the structure of both MOFs. The dia-gram also reveals an area at the intersection of all three structures, that represents the residual frus-tration. While an undistorted paddlewheel was ob-served, no structure with all carboxylic acids binding to copper or no accessible voids was observed. The structures reported here point towards using geo-metrically frustrated assembly as a design principle in reticular materials to form metal containing HOFs, unusual metal SBUs, dangling functional groups and polymorphic structures.

ASSOCIATED CONTENT Supporting Information The Supporting Infor-mation is available free of charge on the ACS Publi-cations website at DOI:. Materials and methods; Supplementary Figures S1−S23 containing SRXD data, TGA and additional in-formation about the nets. (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

Author Contributions F.H. and S.F. designed the project. F.H. performed all experiments. F.H. and H.S. measured the single crys-tal diffraction and solved the crystal structures. G.A.C and M.B performed the sorption experiments. F.H. and S.F wrote the manuscript and all authors commented on the manuscript.

Funding Sources

Notes

The authors declare no competing financial inter-ests.

ACKNOWLEDGMENT F.H. is grateful to the Japan Society for the Promo-tion of Science (JSPS) for Post-Doctoral Fellowships. The authors acknowledge iCeMS Analysis Center for access to analytical facilities.

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Double−Step Gas Sorption of a Two−Dimensional Metal−Organic Framework. J. Am. Chem. Soc. 2007, 129 (41), 12362–12363. https://doi.org/10.1021/ja073568h.

(31) Li, J.-R.; Yu, J.; Lu, W.; Sun, L.-B.; Sculley, J.; Balbuena, P. B.; Zhou, H.-C. Porous Materials with Pre-De-signed Single-Molecule Traps for CO2 Selective Ad-sorption. Nat. Commun. 2013, 4, 1538. https://doi.org/10.1038/ncomms2552.

(32) Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database. Acta Crystal-logr. Sect. B Struct. Sci. Cryst. Eng. Mater. 2016, 72 (2), 171–179. https://doi.org/10.1107/S2052520616003954.

(33) Gong, W.; Chu, D.; Jiang, H.; Chen, X.; Cui, Y.; Liu, Y. Permanent Porous Hydrogen-Bonded Frameworks with Two Types of Brønsted Acid Sites for Hetero-geneous Asymmetric Catalysis. Nat. Commun. 2019, 10 (1), 1–9. https://doi.org/10.1038/s41467-019-08416-6.

(34) Wong-Foy, A. G.; Lebel, O.; Matzger, A. J. Porous Crystal Derived from a Tricarboxylate Linker with Two Distinct Binding Motifs. J. Am. Chem. Soc. 2007, 129 (51), 15740–15741. https://doi.org/10.1021/ja0753952.

(35) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; How-ard, J. a. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Pro-gram. J. Appl. Crystallogr. 2009, 42 (2), 339–341. https://doi.org/10.1107/S0021889808042726.

(36) Sheldrick, G. M. A Short History of SHELX. Acta Crys-tallogr. A 2008, 64 (1), 112–122. https://doi.org/10.1107/S0108767307043930.

(37) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr. Sect. C Struct. Chem. 2015, 71 (1), 3–8. https://doi.org/10.1107/S2053229614024218.

(38) Spek, A. L. PLATON SQUEEZE: A Tool for the Calcu-lation of the Disordered Solvent Contribution to the Calculated Structure Factors. Acta Crystallogr. Sect. C Struct. Chem. 2015, 71 (1), 9–18. https://doi.org/10.1107/S2053229614024929.

(39) Spek, A. L. Structure Validation in Chemical Crystal-lography. Acta Crystallogr. D Biol. Crystallogr. 2009, 65 (2), 148–155. https://doi.org/10.1107/S090744490804362X.

(40) Blatov, V. A.; Shevchenko, A. P.; Proserpio, D. M. Ap-plied Topological Analysis of Crystal Structures with the Program Package ToposPro. Cryst. Growth Des. 2014, 14 (7), 3576–3586. https://doi.org/10.1021/cg500498k.

8

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1

SUPPORTING INFORMATION

NODE DISTORTION IN THE HYDROGEN BONDED NODES

The packing of the Cu2(pLH4)4 molecules in the solid state allows all free carboxylic acids to participate in

hydrogen bonding and enables hydrophobic interaction between the pi-systems of the pL linkers, which likely

additionally stabilize the structure. In pentaMOF-3, one carboxylic acid per [pLH]4- unit participates in hydro-

gen bonding instead of binding to a copper SBU, thereby replacing a node with four points of extension (copper paddlewheel) with a node with two points of extension (hydrogen bonding). In pentaMOF-2, one carboxylic

acid per [pLH]4- unit is dangling into the solvent filled void, not participating in any hydrogen bonding to the

MOF backbone. In the pentaHOF-1 structure, four carboxylic acids are closely grouped in such a way that they

can only form three instead of the optimal four hydrogen bonds. The orientation of the free carboxylic acids is determined by the in the copper paddlewheel at the center of the molecule. Packing this rigid molecule pre-

cludes the formation of the favored dimer structure with two hydrogen bonds per dimer. At the same time,

transforming the group of four free carboxylic acid groups into another copper paddlewheel is not expected,

as it would lead to an extremely strained coordination environment. This causes the hydrogen bonded motif to be formed instead of a copper bonded SBU. In pentaMOF-3 the hydrogen bonded carboxylic acid dimer is only

able to form a single hydrogen bond due to the offset of the carboxylic acids that is determined by the metal

ligand bonded framework.

GEOMETRIC FRUSTRATION IN OTHER FRAMEWORK MATERIALS

Anomalous structures that reveal geometric frustration have been in principle found before, but not been

discussed in this context or systematically investigated.

One example is the work by Gong et al.33 where a highly distorted tetrahedral tetracarboxylic acid was com-bined with a tetrapod calixarene type metal node. The tetrapod nature of the metal node leads to difficulty in

translating this motif into three dimensions, due to the acute angles between adjacent linkers in combination

with the incompatible geometry of the organic 4-c linker. As a result, a metal organic polyhedron is obtained

from the tetrapod linker with three out of four carboxylic acid groups. This is an example of self-limiting assembly as a molecular cage is obtained, that still possesses dangling carboxylic acids. These groups then

participate in hydrogen bonding with other cage molecules to form a crystalline lattice, similar to pentaHOF-

1.

A second examples is the work by Wong-Foy et al.34 as in the formation of a MOF with untypical copper metal SBU due to incompatibilities between the linker and the typical paddlewheel SBU. In this work two types of

nodes are observed in one structure, one being the typical copper paddlewheel and the other being an unusual

Cu3(O2CR)6 cluster. This cluster is formed due to the desymmetrized design of a 3-c linker based on biphenyl

tricarboxylic acid. This can be understood as an internally stressed equilibrium structure, as the not all SBU

can form the typical copper paddlewheel.

METHODS 1H and 13C NMR spectra were recorded on a Bruker Biospin DRX - 600 (600 MHz) spectrometer at 25 °C. IR

spectra were recorded in ATR geometry on a Jasco FT/IR-6100 spectrometer. TGA was performed on a Rigaku model Thermo plus EVO under a Nitrogen atmosphere with a heating rate of 5°C min−1. PXRD data was collected

using a Rigaku SmartLab diffractometer with Cu Kα radiation (λ = 1.54056 Å) in Bragg-Brentano geometry. Gas

sorption isotherms were measured volumetrically with a BELSORP-max (BEL Japan, Inc) at 77 K (N2) and 195

K (CO2). Samples were activated for 12 hours under vacuum at a temperature of 120°C. Single crystal X-ray diffraction data was obtained on a Rigaku model XtaLAB P200 diffractometer equipped with a Dectris model

PILATUS 200K detector and confocal monochromated Mo Kα radiation (λ = 0.71075 Å). The structures were

solved by intrinsic phasing using ShelXT as implemented in within Olex235 using least squares minimization

(Table S1).36,37 All structures required the used of the SQUEEZE38 routine implemented within PLATON39 to

model disordered solvents.

2

SYNTHESIS

Figure S1. Overview of the synthesis of the pLH5 linker.

1- (4 –(methyl carboxylate) phenyl) pyrrole (1) was synthesized based on a previously reported method:

Methyl 4-aminobenzoate (3.023 g, 20 mmol) and 2,5-Dimethoxytetrahydrofuran (2.97 ml, 22 mmol) were se-quentially added to acetic acid (15 ml) in a 20 ml microwave vial. The vial was sealed and heated to 150°C for

10 min in a Biotage© Initiator microwave. After cooling down, the reaction mixture was poured into 150 ml of

ice-cold water. The resulting suspension was extracted with dichloromethane (DCM, 100 ml). The organic

phase was washed with saturated sodium bicarbonate solution (100 ml) and water (2x 100 ml). The organic fractions were dried with MgSO4 and removed under reduced pressure. The solid was purified by flash chro-

matography on silica gel in pure DCM. The product was obtained as a white solid (3.3 g, 82 % yield). 1H NMR

(ppm, CDCl3): 8.1242 (d, 2H), 7.4713 (d, 2 H), 7.1859 (t, 2H), 6.4112 (t, 2H), 3.9560 (s, 3H); 13C (ppm, CDCl3):

166.4211, 144.0212, 131.3160, 126.9666, 119.3176, 119.0389, 111.4980, 52.1451. HRMS calcd. for C12H12NO2

[M+H]+ 202.0863, found 202.0861.

3

1- (4 –(methyl carboxylate) phenyl) 2,3,4,5 tetrabromo pyrrole (2): 1 (2.012 g, 10 mmol) was dissolved in

28 ml dry dimethyl formamide (DMF) this solution was cooled to 0°C and a solution of N-bromo succinimide (7.119 g, 40 mmol) dissolved in DMF (14 ml) was added dropwise. After the complete addition, the mixture

was stirred for additional 30 minutes at 0°C and poured into 100 ml of ice-cold water. The aqueous mixture

was extracted with chloroform (3 x 100 ml). The organic phase was washed with water (3x 100 ml) and dried

with MgSO4. The solvent was removed under reduced pressure and the obtained solid purified by flash chro-matography on silica gel in 40% chloroform in hexane. The product was obtained as a white solid (2.15 g, 42

% yield). 1H NMR (ppm, CDCl3): 8.2129 (d, 2H), 7.3500 (d, 2H); 3.9887 (s, 3H); 13C (ppm, CDCl3): 165.38731,

141.6853, 131.6191, 130.6347, 128.7737, 104.2093, 103.4943, 52.5420. HRMS calcd. for C12H8Br4NO2 [M+H]+

513.7283, found 513.7281.

1,2,3,4,5 - Penta (4 – (methyl carboxylate) phenyl) pyrrole (3): 2 (5.168 g, 10 mmol), 4-methoxycarbon-

ylphenylboronic acid (10.798 g, 60 mmol), Potassium phosphate (8.491g, 40 mmol), bis (triphenylphosphine)

palladium(II) dichloride (0.351 g, 0.5 mmol), tetrakis (triphenylphosphine) palladium(0) (0.577 g, 0.5 mmol) were sequentially added to a solution of 200 ml deoxygenated dioxane and 50 ml water while degassing the

solution by bubbling Argon. After 30 minutes the reaction mixture was heated to 120°C for 16h. After the

reaction mixture was cooled to room temperature, DCM (400 ml) and water (300 ml) were added and ex-

tracted. The organic phase was dried over MgSO4 and the solvent removed under reduced pressure. Ethanol (500 ml) was added to the resulting solid and mixed thoroughly. The remaining solid was filtered off and

purified by flash chromatography on silica gel in 5% EtOAc in DCM. The product was obtained as a white solid

(4.2 g, 57 % yield). 1H NMR (ppm, CDCl3): 7.8478 (d, 2H), 7.7731 (d, 4H), 7.7562 (d, 4H), 6.9684 (d, 2H),

6.9415 (d, 4H), 6.9252 (d, 4H), 3.8895 (s, 3H), 3.8677 (s, 6H), 3.8644 (s, 6H); 13C (ppm, CDCl3): 166.972, 166.5958, 165.9894, 141.5631, 139.0282, 135.5969, 131.8422, 130.9748, 130.7225, 130.1873, 129.3499,

129.3213, 129.2489, 128.8514, 128.7838, 127.9588, 123.6447, 52.3136, 52.1221, 51.9924. HRMS calcd. for

C44H35NNaO10 [M+Na]+ 760.2153, found 760.2153

4

1,2,3,4,5 - Penta (4 – (benzoic acid) pyrrole (pLH5): 3 (2.951 g, 4 mmol) is added to a solution of Sodium

hydroxide (1 g, 25 mmol) in methanol (33 ml) and water (33 ml). The reaction mixture was heated to 80°C for

5 h. After cooling down, the reaction mixture was poured into ice cold water (200 ml) and was extracted with

ethyl acetate (200 ml). The aqueous layer was acidified with concentrated hydrochloric acid until pH 1. The resulting solid is separated by centrifugation and repeatedly redispersed and washed with water (3x 100 ml).

The wet solid is dissolved in ethanol and the liquid dried in air to obtain the pure solid as a slightly yellow

powder (2.2 g, 82 % yield). 1H NMR (ppm, DMSO-d6): 12.9309 (s, 5H), 7.7687 (d, 2H), 7.6948 (m, 8H), 7.1881

(d, 2H), 7.0535 (d, 4H), 6.9941 (d, 4H); 13C (ppm, DMSO-d6): 167.5721, 167.3186, 166.9081, 141.4393, 139.1803, 135.6250, 132.2470, 131.5594, 131.0072, 130.5109, 130.2410, 129.9654, 129.7360, 129.4821,

129.3376, 128.9436, 123.1447. HRMS calcd. for C39H24NO10 [M-H]- 666.1406, found 666.1404

GROWTH OF SINGLE CRYSTALS

pentaHOF-1 single crystals were grown by layering a solution of Cu(NO3)2•6H2O (18.1 mg, 0.075 mmol) in 0.5 ml water with first a buffer layer (0.5 ml water and 0.5 ml ethanol) and then with a solution of pLH5 (20.0

mg, 0.03 mmol) in ethanol (0.5 ml) at room temperature. Large millimeter sized blue block shaped crystals

grew after 10 days that were suitable for single crystal X-ray diffraction.

pentaMOF-2 single crystals and bulk power was obtained by dissolving Cu(NO3)2•6H2O (0.1812 mg, 0. 75 mmol) and pLH5 (200.0 mg, 0.3 mmol) in a mixture of DMF (10 ml) and n-butanol (10 ml) and heating the

solution at 40°C for 7 days. Sub-millimeter sized turquoise block shaped crystals were formed that were suit-

able single crystal X-ray diffraction.

pentaMOF-3 single crystals were grown by layering a solution of Cu(NO3)2•6H2O (18.1 mg, 0.075 mmol) in water (1 ml) with a solution of pLH5 (20.0 mg, 0.03 mmol) in water saturated n-butanol (1 ml) in a glass

pressure tube. The reaction vessel was heated to 110°C for 66h with 3h ramp up and a 1 h ramp down. Sub

millimeter sized green block shaped crystals were formed that were suitable single crystal X-ray diffraction.

SYNTHESIS OF BULK MATERIALS

pentaHOF-1 was synthesized as a bulk powder by mixing a solution of Cu(NO3)2•6H2O (120.8 mg, 0.5 mmol) in water (6.7

ml) and pLH5 (133.5 mg, 0.2 mmol) in ethanol (6.7 ml). The mixture was heated in a screw cap vial to 90°C for

24 h with a 6 h ramp to room temperature. The blue crystalline solid was filtrated and washed with 5ml of a

1:1 mixture of water and ethanol, 5 ml of ethanol and dried in air. Elemental Analysis: found, H: 4.65, C: 60.32,

N: 1.71; expected, H:3.46, C:67.07, N: 2.01.

pentaMOF-2 single crystals and bulk power was obtained by dissolving Cu(NO3)2•6H2O (0.1812 mg, 0. 75

mmol) and pLH5 (200.0 mg, 0.3 mmol) in a mixture of DMF (10 ml) and n-butanol (10 ml) and heating the

solution at 40°C for 7 days. The turquoise crystalline solid was filtrated and washed with 10ml of a 1:1 mixture of DMF and n-butanol. The sample was activated by repeatedly immersing the powder in diethyl ether and

exchanging the liquid with fresh diethyl ether every day for 7 days. Elemental Analysis: found, H: 3.32, C:

56.25, N: 2.18; expected, H:2.68, C:59.24, N: 1.77.

pentaMOF-3 was synthesized as a bulk powder by first creating a solution of pLH5 (100 mg, 0.15 mmol) in 1-propanol (4 ml), by sonication and addition of water dropwise until a clear solution is obtained. To this solution

is added 1M NaOH (0.3 ml, 0.3 mmol) and thoroughly mixed, resulting in a white precipitate. This dispersion

is added to a solution of Cu(NO3)2•6H2O (72.5 mg, 0.3 mmol) in water (4 ml) and briefly mixed. The reaction

mixture is then heated to 60°C for 24 h and cooled down with a 2°C/h ramp to room temperature. The powder is filtered off and washed with a small amount of 1:1 mixture of water and 1-propanol and briefly sucked dry.

Elemental Analysis: found, H: 3.88, C: 50.92, N: 1.93; expected, H:2.68, C:59.24, N: 1.77.

SXRD REFINEMENT

Guest molecules that could not fully be modelled and refined were removed, void spaces occupied by disor-

dered solvent molecules were treated with the squeeze routine in Platon. The position of the nitrogen was

5

located by refining the model with a five membered ring composed of only carbon where the occupancy was

left open to refine. The carbon atom with the highest occupancy was replaced by a nitrogen for the final model.

Table S1: Crystal data and structure refinement.

Compound Name pentaHOF-1 pentaMOF-2 pentaMOF-3

Wavelength (Å) 0.71073 0.71073 0.71073

T (K) 100 100 100

Crystal System Tetragonal Monoclinic Orthorhombic

Space Group P4/nnc C2/c Pna21

a (Å) 32.9819(4) 24.4730(6) 28.6889(3)

b (Å) 32.9819(4) 27.0553(4) 21.1660(2)

c (Å) 9.9569(2) 20.2370(5) 14.4751(2)

α (°) 90 90 90

β (°) 90 113.928(3) 90

γ (°) 90 90 90

V (Å3) 10831.2(3) 12247.8(5) 8789.70(17)

Z 2 1 8

Dcalc (g cm-3) 0.861 1.286 1.198

Reflections 7184 34652 23585

Unique Data 5769 16383 18962

Rint 0.0264 0.0194 0.0304

R[F2>2σ(F2)] 0.0800 0.0773 0.0564

wR(F2) 0.2532 0.2458 0.1680

S 1.039 1.041 1.041

ρmax, ρmin (eÅ-3) 1.125, -0.930 1.433, -1.346 1.877, -0.569

NETS

The nets were determined using the ToposPro40 software. In the case of pentaMOF-2 the rod SBU was

described as a node with six points of extension that connect to each other forming the rod SBU. The

topology obtained are therefore sqc54 for pentaMOF-3 and sqc176 (lvt) for pentaMOF-2.

6

Figure S2. Hydrogen bonding network in pentaHOF-1 (bct)

Figure S3. Visualization of the 10 next neighbors of one Cu2(pLH4)4 molecule in pentaHOF-1.

Figure S4. Hydrogen bonding in pentaHOF-1 (a, b) and pentaMOF-3 (c).

7

Figure S5. Solvent acessible voids in pentaHOF-1 with a 0.8 Å probe radius. Seen along c.

Figure S6. Location of the solvent molecules in between the linkers in pentaMOF-2, as determined by SXRD.

8

Figure S7. Solvent acessible voids in pentaMOF-2 with a 0.8 Å probe radius. Seen along c.

Figure S8. Schematic of the pentaMOF-3 net (sqc54).

9

Figure S9. Solvent acessible voids in pentaMOF-3 with a 0.8 Å probe radius. Seen along c.

Figure S10. Distortion of the copper paddlewheel in pentaMOF-3 due to the linker overlap (top). Schematic of

the formation of the layer, leading to the strain (bottom).

10

Table S2: Bending of the pLH5 linker determined as angles between the mean plane of the pyrrole

core and the carbon atom of the carboxylic acid in pentaMOF-3.

Out of plane angle (°) Type of Carboxyphenly group

2.711336 Hydrogen bonded

11.35441 Paddlewheel

0.381753 Rod SBU

1.911587 Paddlewheel

0.156524 Rod SBU (bridging)

1.500497 Rod SBU

1.523784 Paddlewheel

6.197691 Rod SBU

0.948649 Hydrogen bonded

12.82044 Paddlewheel

2.059116 average Rod SBU

6.902555 average Paddlewheel

1.829993 average Hydrogen bonded

3.303123 average [pLH]4- with 1 bridging carboxylate in the rod SBU

4.598212 average [pLH]4- with 0 bridging carboxylate in the rod SBU

Figure S11. Details on the qualitative evaluation of frustration in the penta-framework series.

11

Figure S12. Microscopy image of pentaHOF-1 single crystals.

Figure S13. Microscopy image of pentaMOF-2 single crystals.

Figure S14. Microscopy image of pentaMOF-3 single crystals.

12

5 10 15 20 25 30 35 40 45 50

2 theta (°)

Inte

nsity (

a.u

.)

pentaHOF-1 as synthesized

pentaHOF-1 simulated

Figure S15. XRD of the as synthesized and simulated pentaHOF-1.

5 10 15 20 25 30 35 40 45 50

Inte

nsity (

a.u

.)

pentaMOF-2 as synthesized

pentaMOF-2 simulated

2 theta (°)

Figure S16. PXRD of the as synthesized and simulated pentaMOF-2.

5 10 15 20 25 30 35 40 45 50

pentaMOF-3 as synthesized

pentaMOF-3 simulated

2 theta (°)

Inte

nsity (

a.u

.)

Figure S17. PXRD of the as synthesized and simulated pentaMOF-3.

13

5 10 15 20 25 30 35 40 45 50

No

rma

lize

d in

ten

sity (

a.u

.)

2 theta (°)

pentaHOF-1 after activation

pentaHOF-1 as-synthesized

Figure S18. PXRD of pentaHOF-1 before and after activation.

10 20 30 40 50

Inte

nsity (

a.u

.)

2 theta (°)

pentaMOF-2 after activation

pentaMOF-2 solvent exchanged

Figure S19. PXRD of pentaMOF-2 after solvent exchange and after activation.

14

5 10 15 20 25 30 35 40 45

No

rma

lize

d in

ten

sity (

a.u

.)

2 theta (°)

pentaMOF-3 after activation

pentaMOF-3 as-synthesized

Figure S20. PXRD of pentaMOF-3 before and after activation.

0 100 200 300 400 500

0

20

40

60

80

100

-30.83 %

Temp (°C)

TG

(%

)

-11.32 %

0

10

20

30

40

DT

A (µ

V)

Figure S21. TGA of as-synthesized pentaHOF-1.

15

0 100 200 300 400 500

-60

-40

-20

0

25.93 %

Temp.(°C)

TG

(%)

29.78 %

-10

0

10

20

30

40

50

DT

A(µ

V)

Figure S22. TGA of as-synthesized pentaMOF-2.

0 100 200 300 400 500

0

20

40

60

80

100

Temp (°C)

TG

(%

)

0

10

20

30

DT

A (µ

V)

-34.31 %

-14.96 %

Figure S23. TGA of as-synthesized pentaMOF-3.

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