Interpenetrated Frameworks with Anisotropic Pore Structures from a

Article pubs.acs.org/crystal

Interpenetrated Frameworks with Anisotropic Pore Structures from a Tetrahedral Pyridine Ligand Florian L. Geyer,† Frank Rominger,† Maximilian Vogtland,† and Uwe H. F. Bunz*,†,‡ †

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Centre of Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

‡

S Supporting Information *

ABSTRACT: Herein we investigate metal−organic frameworks (MOFs) employing the tetrahedral building block tetrakis(4-(pyridin-4-ylethynyl)phenyl)silane (1) as a structure-generating feature. Combining 1 with a series of different metal ions gives wildly different MOF topologies, porosities, and properties. The frameworks discussed deviate from the common construction principles of MOFs as both the ligand and the metal center impose their coordination geometry on the overall network. The resulting networks show attractive and unusual void distributions, yet with properly controlled transition metal ion geometries, these MOFs show a rational structural grammar with respect to their structures.

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INTRODUCTION Today, research on metal−organic frameworks (MOFs) is focused more upon function and less on the discovery of new structures. Consequently, the construction of large classes of potent ligands remains unstudied. We were interested in a new approach to prepare structures based on pyridine frameworks, and thus we synthesized the tetrahedral, expanded pyridine ligand 1.

imposes the coordination geometry of the metal center on the overall crystal. Yaghi’s zincate−carboxylic acid MOFs demonstrate this strategy powerfully.1,5 While the coordination of the cluster dictates the overall network topology, the length of the carboxylic acid rod determines the porosity and pore size and, as a consequence, often the degree of interpenetration.6,7 The envisioned application (e.g., gas storage) of many frameworks is directly connected to a high porosity thatin order to be stable against the removal of solvent from the poresrequires rigid, strong coordinative bonds, i.e., carboxylic acid ligands, which usually also lead to internally electroneutral networks without free or dangling anions. The combination of the “node and linker” principle with dicarboxylic acids has led to new structures, which still dominate the research on MOFs. As the field rapidly matures, the development of functional carboxylic acid MOFs rather than the discovery of new structures becomes the focus.8,9 Recent trends are the development of catalytically active MOFs,10,11 new materials for separation and extraction,12 and nanoscaling MOFs for drug-delivery applications, as pioneered by the research groups of Lin and Eddaoudi.13−15 Probably because of this dominance of carboxylic acid MOFs, other classes of coordination polymers remain largely undeveloped.16 One of these classes comprises pyridine-based frameworksprobably also due to the relative lability of the coordinative bond pyridine forms with transition metals and the presence of unattached anions. On the other hand, pyridine

In the early stages of MOF chemistry, the focus was on the exploration of the diverse and often aesthetic crystal structures.1,2 While the structural diversity and beauty of the structures remain stunning, guidelines for the design of MOFs (predetermination of the networks) and the encoding of structures were soon established. The most important and most frequently applied strategy toward this goal, referred to as “node and linker” principle,3 was already envisioned by Hoskins and Robson in 1990.4 Co-crystallization of organic and inorganic linkers into a defined crystalline framework is a multiparameter process employing multinodal ligands as well as coordination centers further complicates predicting the resulting structure. Using linear, bifunctional molecular rods (ligands) together with the well-defined coordination chemistry of transition metal ions or clusters simplifies the prediction of a crystal structure, as it © XXXX American Chemical Society

Received: May 26, 2015 Revised: May 30, 2015

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DOI: 10.1021/acs.cgd.5b00719 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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RESULTS AND DISCUSSION Twenty-Fold Interpenetrated Framework with Ag(I) Nitrate. As expected, AgNO3 connects with 1 to form an expanded, diamondoid network (Figure 2). Looking at a single

coordination chemistry is exceptionally rich, and thus, a huge variety of networks should be realized. Interpenetration can be a valid strategy to both mechanically stabilize MOFs and also fine-tune the pore size, shape, and distribution throughout the material7at the price of diminishing the overall pore volume of the network. In this regard, “simple” networks (like diamondoid nets) will result in a high degree of interpenetration upon elongation of the connecting rods. Excessive interpenetration results in nonporous solids, as elegantly shown by Ermer in 1988.17 Complex networks formed from two or more nodes instead of one node and rod-like ligands usually cannot stack densely; free volume remains after network interpenetration. Our approach thus aims to encode the network topology within both components of the framework. In this regard, it is related to a strategy recently presented by Eddaoudi et al., where highconnectivity building blocks are applied to allow only for the formation of the desired network topology.18,19 In contrast, our low-connectivity building blocks do not preclude interpenetration but rather feature it to produce the pore structure and further stabilize the pyridine network. We were interested in the structures obtained by cocrystallizing tetrakis(4-(pyridin-4-ylethynyl)phenyl)silane (1, Figure 1) with a variety of transition metal salts. While ligand

Figure 2. Diamondoid framework formed from 1 and AgNO3. (A) One adamantane unit of the network. (B) Two representatives of 10 translationally dependent networks. (C) Two representatives of the two sets of symmetry-independent networks. (D) In total, 20 networks are interpenetrated, forming a densely packed solid consisting of two pairs of 10 translationally independent networks each.

network, 97% void space would ariseaccordingly, the network interpenetrates to minimize free volume. Due to the slim rods, represented by two “arms” of the ligand and one Ag(I), a very high,22,23 20-fold interpenetration results. The interpenetration is to be understood as two symmetryindependent pairs of 10 separate networks each. This stabilizes the network to a point where the crystals can be easily removed from the solvent and subjected to X-ray analysis. The nitrates are not localized in the crystal. Squeezing the non-localized electron density from the diffraction data gives some insight into the remaining porosity. Per 22 297 Å3 unit cell, 3625 non-localized electrons constitute a volume of 8192 Å3 (37% non-crystalline volume). A similar value (8281 Å3) is obtained by calculating the vdW surface of the unit cell,27 showing the tight encapsulation of the solvent molecules in the crystal. Of this volume, 2048 Å3 belongs to the nitrates, resulting in a residual pore volume of 28%. Only 7% of the unit cell volume is taken up by pores large enough to contain gas molecules; however, the pores are not interconnected and thus are inaccessible. Highly Porous PtS Network with Cu(II) Nitrate. The framework afforded from 1 plus Cu(NO3)2 crystallizes in several millimeter-sized, blue needles (Figure 3). X-ray analysis reveals the network to be the simplest combination of

Figure 1. Molecular structure of tetrakis(4-(pyridin-4-ylethynyl)phenyl)silane (1) in the crystal.

1 does form crystalline solids with many transition metal salts, the structures discussed below could be determined by X-ray analysis. From the viewpoint of coordination chemistry, the ligand is closely related to tetra(4-pyridinyl)methane;20 however, the additional phenyl ethynyl units significantly enlarge the “struts” of the ligand. Looked at naively, this should result in enlarged pores but, in realityas often observedwill result in increasingly interpenetrated networks.21 As we impose both the tetrahedral geometry of the ligand and the coordination geometry of the metal center on the network, interpenetration to such a degree that all porosity is lost should only be possible for linearly coordinating metal ions. Other coordination geometries, e.g., square-planar, tetrahedral with co-ligands, trigonal etc., should result in more complex networks where interpenetration to an essentially non-porous material will no longer be possible and porous structures can be expected. B



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the Zn(II) center impose a tetrahedral geometry (Figure 4). Two of the ligands coordinating to the inner sphere of the Zn2+

Figure 4. Framework formed from 1 and Zn(OAc)2: (A) top view and (B) side view. (C,D) Schematic representation of the eight-membered ring from the 2D network. (E) Stacking of these networks results in (F) channels parallel to the c-axis.

cations are acetates. The tetrahedrally coordinated zinc cation thus connects only two pyridines through the remaining free coordination sites. This results in a two-dimensional network a layer out of a diamondoid net. The layers do not interpenetrate. Instead, they stack together as described in Figure 4. The only accessible void spaces left are noninterconnected channels aligned parallel to the c-axis. The channel structure is a direct consequence of the stackingwith simple tetrahedral linkers connected to a 2D framework, a highly anisotropic pore-distribution can be achieved by the “correct” stacking of the layers. Considering the strong coordination of the acetatesthey do not easily move into the second coordination sphere as do the nitrates in the Ag MOF (Figure 2) or the nitrates in the Cu(II)-PtS MOF (Figure 3)their direct coordination to the zinc cation could be expected. The two-dimensional layer structure is the only ordered structure to be built from these linkers. The 2D layer also allows for minimization of the void space. A disordered 3D network could be imagined as a diamondoid net with “missing connections”. However, stacking of the 2D networks is much more space-efficient. Due to the long struts of the ligands, even with dense stacking of the sheets onto each other, void space remains in form of tunnels. The tunnels are thus a consequence of the materials striving to minimize free volume during the crystallization process. This approach should be generalizable for the construction of

Figure 3. Crystal structure of the framework formed from 1 and Cu(NO3)2. (A) Single network segment visualizing the PtS structure. Anions have been omitted for clarity. Visualization of the two-fold network interpenetration, viewed (B) down the c-axis and (C) on the c-axis. Voids (D) down the c-axis and (E) on the c-axis.

tetrahedral and square-planar knots, i.e., a PtS network. This is the network to be expected from the “quasi”-square-planar Cu(II) (two nitrates in the apexes of an octahedral sphere, four spaces left for other ligands that arrange in-plane). The PtS network is relatively rare within the large group of MOFs based on four-fold connectors.24−27 Due to the long struts of the tetrapyridines, much void space is left in a single network; however, the structure only allows for a two-fold interpenetration. The resulting crystal is porous, with a calculated28 gas-accessible pore volume of 67% and a surface area of 1273 m2/g. Large channels pierce the crystal from all directions, thus making the void space fully accessible. As expected for a pyridine net with that much free space, the crystals collapse upon drying and thus are not suitable for gas sorption experiments. They do, on the other hand, allow for exchanging the solvent and can be analyzed by X-ray at low temperature. 2D Framework with Zn(II) Acetate. In the framework formed from 1 with Zn(OAc)2, both the pyridine ligand and C



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Cu(II) in the Cu(NO3)2 network with 1, should be possible, and the two-fold interpenetrated PtS net may not remain the most stable form. Instead, linear Cu(II) dimers may stabilize an interpenetrated network comparable to the AgNO3 framework presented above. This is not the case. While the system 1 plus Cu(OAc)2 does stabilize itself by crystallizing in a network more capable of interpenetration, several differently coordinated copper linkers are created, resulting in a complex crystal structure. Its chemical makeup is depicted in Chart 1.

materials with aligned and possibly functionalized tunnels. The dense packing also stabilizes the networkdespite the remaining 7% calculated void space, the material did not disintegrate upon removal (dried on air) of the solvent for Xray crystallography. Complex Framework Formed with Cu(II) Acetate. Acetate ligands are more strongly coordinating than nitrates; the carboxylate group also stabilizes late transition metal dimers. While zinc(II) is not capable of forming these dimeric structures under the mild crystallization conditions (see Supporting Information), copper(II) acetate itself is dimeric in its solid-state structure, and dimers within coordination compounds are well known (Figure 5).29 The availability of dimeric structures and several coordination modes of the acetate itself induces coordination-chemical flexibility in the Cu(II) system. Several linkers, apart from the square-planar

Chart 1. Cu(II)-Pyridine Linkers and Their Assembly into the Central, Dimeric Building Block of the Networka

a

Three such building blocks constitute a six-membered assembly, representing the basic structural motif of the crystal formed from 1 and Cu(OAc)2.

Linker a is a linear monomeric copper(II) complexed by two pyridines and two acetates, of which one is coordinating with both oxygens while the other one is only monodentate. Overall, the coordination of the Cu(II) appears trigonal bipyramidal. The linear linker b is a copper(II) dimer stabilized by four bidentate coordinating acetatesa motif found in the crystal structure of copper(II) acetate itself. Linker c is a pseudooctahedrally coordinated copper(II). Three pyridines coordinate in one plane, while two acetates coordinate perpendicular to this plane, resulting in a T-shaped linker from a topological point of view. Located relatively close in the crystal, but outside of the vdW radii, is a free pyridine d, sterically shielding the apical coordination site of the Cu(II) (Figure 5C). If connected to 1, a dimeric “ethane-like” building block is obtained. This building block assembles into a complex 3D network with rings containing two, four, and six pyridine tetrahedrons. An important structural element is the sixmembered assembly depicted in Chart 1. As the free pyridine d is not coordinated to linker c, the rings are not closed, yet the network it constitutes is best described as a 6,3-network.

Figure 5. Framework formed from 1 and Cu(OAc)2: (A) side view and (B) top view. Hexagonal assembly (compare Chart 1): (C) side view into the network visualizing the free pyridines and layer connections. Visualization of two of the nine intercalated networks, viewed (D) perpendicular to the hexagonal nets and (E) perpendicular to the internetwork connection and free pyridines. Voids down (F) the a-axis and (G) the c-axis. D



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scope for introducing functionality into a given structure. The expanded networks formed from ligand 1 are stabilized through interpenetration thatdue to the multinodal topologyresults in intriguing pore structures in the crystal. The pores are constructed following a grammar determined only by the applied linker 1 and the metal salt and have a surface comprising not only the linking moieties but also the tetraphenylsilane cores. This is in contrast to frameworks constructed from short, rodlike pyridine ligands, e.g., di(4pyridyl)acetylene, and opens up further possibilities for the construction of interesting materials.

Combining these structural elements results in a layered, “hexagonal” networkeach net is interconnected to the next by the trigonal linker c. This network is nine-fold interpenetrated, minimizing the void space and stabilizing the crystal (calculated gas-accessible void space (Figure 5F,G) to 21%). The pores’ alignment (Figure 5F,G) is highly anisotropic and, in that way, comparable to that of the Zn(OAc)2 network. The void space is layered into planes perpendicular to the caxis. In these planes, columnar tunnels are aligned parallel to the a-axis. Every second tunnel pierces the whole crystal; the other is separated into smaller sections. Both types of tunnels are interconnected in such a way that the whole void-layer is accessible. Giving the Cu(II) cations coordinative flexibility while changing from the nitrate to the acetate greatly influences the crystal structure. The complex and unexpected network formed from 1 and Cu(OAc)2 further stabilizes itself by nine-fold interpenetration. As the network itself cannot interpenetrate in a way that eliminates all pores, channels are formed.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, crystallization conditions, spectral data, and crystal structures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00719.

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CONCLUSIONS While most research in the area of coordination polymers focuses on rigid networks formed from zincate or related clusters with dicarboxylic acid rods, networks formed from other ligands like pyridines as well as networks constructed from more than one kind of node are somewhat neglected. Looking at the crystal structures formed with 1, a rational prediction of the networks is possible as long as the coordination geometry of the transition metal is strictly defined. This is the case for AgNO3 and Cu(NO3)2, where the weakly coordinating nitrate cannot support dimeric copper species, and Zn(OAc)2, where dimeric species or clusters cannot form under the reaction conditions and the strongly coordinating acetate remains tightly bound as a monodentate ligand. From the network topology and the strut length of the ligand, the degree of interpenetration can be estimated. As expected, the diamondoid network formed from AgNO3not strictly a multinodal network, but rather an inversion of the “node and linker” principle (organic node, linear metal)is highly (20×) interpenetrated and loses most of its porosity. The PtS network formed from 1 and Cu(NO3)2, on the other hand, cannot easily interpenetrate. Its two-fold entangled structure is highly porous and collapses upon removal of the solvent. The Zn(OAc)2 network compromises both porosity and dense packing to form a stable crystal. Hexagonally packed channels result along the c-axis of the material. A similar pore structure is observed in the Cu(OAc)2 network, where the coordinative flexibility of the salt creates a network that is more prone to intercatenation than the PtS network. The structure of the Cu(OAc)2 network was unpredictablefrom reasoning, only a diamondoid or PtS network would have been expected. We derive that metal−organic frameworks with predictable network topology and pore structure can be prepared from heterocyclic organic nodes with elongated struts, such as 1 and suitable transition metal salts. The key feature of these materials is the determination of the network through two topologically different nodes: the ligands and the metal centers. Application of the “node and linker” principle to heterocyclic ligands usually requires short rods to stabilize the crystal structure.30 This limits the synthetic possibilities for introducing further functionalities into the organic linkers. With a focus on functional materials, the rod and linker approach limits the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.L.G. thanks the Studienstiftung des Deutschen Volkes for support through a Ph.D. scholarship.

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REFERENCES

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Interpenetrated Frameworks with Anisotropic Pore Structures from a

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