Pore topology analysis in porous molecular systems.

Porous molecular materials are constructed from molecules that assemble in the solid-state such that there are cavities or an interconnected pore network. It is challenging to control the assembly of these systems, as the interactions between the molecules are generally weak, and subtle changes in the molecular structure can lead to vastly different intermolecular interactions and subsequently different crystal packing arrangements. Similarly, the use of different solvents for crystallization, or the introduction of solvent vapour, can result in different polymorphs and pore networks being formed. It is difficult to uniquely describe the pore networks formed, and thus we analyse 1033 crystal structures of porous molecular systems to determine the underlying topology of their void spaces and potential guest diffusion networks. Material-agnostic topology definitions are applied. We use the underlying topological nets to examine whether it is possible to apply isoreticular design principles to porous molecular materials. Overall, our automatic analysis of a large dataset gives a general insight into the relationships between molecular topologies and the topological nets of their pore network. We show that while porous molecular systems tend to pack similarly to non-porous molecules, the topologies of their pore distributions resemble those of more prominent porous materials, such as metal-organic frameworks and covalent organic frameworks.

Adsorption properties and gas chromatographic application of a composite surface-layer sorbent with Terephthalic acid-based metal-organic framework.

In this paper, we report on the fabrication of a new material SiO2@MIL-101(Cr), which synthesis is based on the metal-organic framework MIL-101(Cr) and the silica support Chromosorb W. The obtained surface-layer sorbent was analyzed by powder X-ray diffraction analysis, Raman spectroscopy and porosity measurement. The retention regularities and the thermodynamic characteristics of the sorption of volatile organic compounds from the gas phase were studied for the packed column with the surface-layer sorbent SiO2@MIL-101(Cr). The results indicated that the adsorbent-adsorbate interaction in the mesoporous structure of SiO2@MIL-101(Cr) depends not only on the dispersion attractive forces but also on π-π-stacking and specific interactions with hydroxyl groups and metal cations' open coordination sites on the pore surface. In this work, we gave examples of gas chromatograms of isomers of alkanes, arenes, and chlorine derivatives of methane on a packed column with the obtained sorbent. Examples of gas chromatograms of isomers of alkanes, arenes, and chlorine derivatives of methane on a packed column with the obtained sorbent are presented.

Chiral Motifs in Highly Interpenetrated Metal-Organic Frameworks Formed from Achiral Tetrahedral Ligands.

Formation of highly interpenetrated frameworks is demonstrated. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal-organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks' degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63 % of the unit cell volume. The MOFs have chiral features but are formed from achiral components. The chirality is manifested by the coordination chemistry around the metal center, the structure of the helicoidal channels, and the motifs of the individual networks. Channels of both handednesses are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated with the degree of interpenetration 10-, 16-, and 32-fold, respectively. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å×14.8 Å.

Metallization-Prompted Robust Porphyrin-Based Hydrogen-Bonded Organic Frameworks for Photocatalytic CO2 Reduction.

Under topological guidance, the self-assembly process based on a tetratopic porphyrin synthon results in a hydrogen-bonded organic framework (HOF) with the predicted square layers topology (sql) but unsatisfied stability. Strikingly, simply introducing a transition metal in the porphyrin center does not change the network topology but drastically causes noticeable change on noncovalent interaction, orbital overlap, and molecular geometry, therefore ultimately giving rise to a series of metalloporphyrinic HOFs with high surface area, and excellent stability (intact after being soaked in boiling water, concentrated HCl, and heated to 270 °C). On integrating both photosensitizers and catalytic sites into robust backbones, this series of HOFs can effectively catalyze the photoreduction of CO2 to CO, and their catalytic performances greatly depend on the chelated metal species in the porphyrin centers. This work enriches the library of stable functional HOFs and expands their applications in photocatalytic CO2 reduction.

Coordination Properties of Hydroxyisophthalic Acids: Topological Correlations, Synthesis, Structural Analysis, and Properties of New Complexes.

Hydroxyisophthalic acids are valuable polytopic ligands for the design of functional materials based on coordination polymers due to the variety of charges and coordination modes they possess. Herein, we describe the synthesis, thermal stability, nonlinear optical (NLO) and spectroscopic properties of five novel coordination compounds, [K2 L(H2 O)2 ], [MgL(H2 O)2 ] ⋅ 3H2 O, [CaL(H2 O)3 ], [SrL(H2 O)3 ] ⋅ H2 O, [BaL(H2 O)(H2 O)5 ], and one salt, (NH4 )2 L ⋅ 2H2 O, with 4,5,6-trihydroxyisophthalic acid (H2 L), which has not been tested in assembling crystalline coordination networks before. The peculiarities of the structural organization of the compounds were analyzed and compared with those for other hydroxyisophthalates. The coordination properties of hydroxyisophthalic acids were studied from the topological point of view, and a comparative topological analysis of coordination and H-bonded networks was performed. Structural correlations revealed in this study could be useful for the design of hydroxyisophthalate-based coordination networks, including porous metal-organic frameworks, proton conductors, and NLO materials.

Visualization and Quantification of Geometric Diversity in Metal-Organic Frameworks.

With ever-growing numbers of metal-organic framework (MOF) materials being reported, new computational approaches are required for a quantitative understanding of structure-property correlations in MOFs. Here, we show how structural coarse-graining and embedding ("unsupervised learning") schemes can together give new insights into the geometric diversity of MOF structures. Based on a curated data set of 1262 reported experimental structures, we automatically generate coarse-grained and rescaled representations which we couple to a kernel-based similarity metric and to widely used embedding schemes. This approach allows us to visualize the breadth of geometric diversity within individual topologies and to quantify the distributions of local and global similarities across the structural space of MOFs. The methodology is implemented in an openly available Python package and is expected to be useful in future high-throughput studies.

Topology versus porosity: what can reticular chemistry tell us about free space in metal-organic frameworks?

We present the results of a comprehensive geometrical and topological analysis of 3D coordination networks in 33 790 coordination polymers. We have found relations between topological descriptors and free space of the networks, and have revealed topological types that promote high porosity of metal-organic frameworks.

Solid-state 1D → 3D transformation of polynitrile-based coordination polymers by dehydration reaction.

In crystal structures of two chain coordination polymers [M(tcnopr3OH)2(H2O)2] (M = NiII and CoII; tcnopr3OH- = [(NC)2CC(O(CH2)3OH)C(CN)2]-) based on a N,O or N,N'-bridge polynitrile ligand, the parallel chains are connected via, respectively, C[triple bond, length as m-dash]NH-O and O-HO hydrogen bonds between uncoordinated functional groups of the ligand and coordinated water molecules. Upon heating, both solids undergo dehydration accompanied by degradation of their single crystals. Powder X-ray diffraction showed that non-isostructural triclinic single crystals transformed to isostructural monoclinic compounds. The solid-state reaction yielded 3D coordination polymers [M(tcnopr3OH)2] (M = NiII and CoII) based on a N,N',O-connected tcnopr3OH-. Although previously tens of complexes based on tcnopr3OH and similar anions were synthesized and X-ray characterized, none of these contain a tridentate polynitrile ligand. Thus, this study provides evidence that solid-state reactions allow obtaining novel coordination modes of polynitrile ligands. The possible pathways for the transformation of H-bonded networks to 3D coordination polymers are discussed on the basis of the topological approach. Applicability of the topological approach to predict possible networks of solid-state reaction products based on the crystal structures of initial compounds is demonstrated.

Record Complexity in the Polycatenation of Three Porous Hydrogen-Bonded Organic Frameworks with Stepwise Adsorption Behaviors.

Hydrogen-bonded organic frameworks (HOFs) show great potential in many applications, but few structure-property correlations have been explored in this field. In this work, we report that self-assembly of a rigid and planar ligand gives rise to flat hexagonal honeycomb motifs which are extended into undulated two-dimensional (2D) layers and finally generate three polycatenated HOFs with record complexity. This kind of undulation is absent in the 2D layers built from a very similar but nonplanar ligand, indicating that a slight torsion of ligand produces overwhelming structural change. This change delivers materials with unique stepwise adsorption behaviors under a certain pressure originating from the movement between mutually interwoven hexagonal networks. Meanwhile, high chemical stability, phase transformation, and preferential adsorption of aromatic compounds were observed in these HOFs. The results presented in this work would help us to understand the self-assembly behaviors of HOFs and shed light on the rational design of HOF materials for practical applications.

Diverse π-π stacking motifs modulate electrical conductivity in tetrathiafulvalene-based metal-organic frameworks.

We report three electrically conductive metal-organic frameworks (MOFs) based on a tetrathiafulvalene linker and La3+. Depending on the solvent ratios and temperatures used in their solvothermal synthesis, these MOFs crystallize with different topologies containing distinct π-π stacking sequences of the ligand. Notably, their transport properties correlate rationally with the stacking motifs: longer S···S contact distances between adjacent ligands coincide with lower electrical conductivities and higher activation energies. Diffuse reflectance spectroscopic measurements reveal ligand-based intervalence charge transfer bands in each phase, implicating charge delocalization among mixed-valent tetrathiafulvalene units as the dominant mode of transport. Overall, these frameworks demonstrate how tuning the intermolecular interactions in MOFs serves as a route towards controlling their physical properties.

Topochemical Synthesis of Single-Crystalline Hydrogen-Bonded Cross-Linked Organic Frameworks and Their Guest-Induced Elastic Expansion.

Covalently linked single-crystalline porous organic materials are highly desired for structure-property analysis; however, periodically polymerizing organic entities into high dimensional networks is challenging. Here, we report a series of topologically divergent single-crystalline hydrogen-bonded cross-linked organic frameworks (HCOFs) with visible guest-induced elastic expansions, which mutually integrate high structural order and high flexibility into one framework. These HCOFs are synthesized by photo-cross-linking molecular crystals with alkyldithiols of different chain lengths. Their detailed structural information was revealed by single-crystal X-ray analysis and experimental investigations of HCOFs and their corresponding single-crystalline analogues. Upon guest adsorption, HCOF-2 crystals composed of a 3D self-entangled polymer network undergo anisotropic expansion to more than twice their original size, while the 2D-bilayer HCOF-3 crystals exhibit visible, layered sorption bands and form delaminated sheets along the plane of its 2D layers. The dynamic expansion of HCOF networks creates guest-induced porosity with over 473% greater volume than their permanent voids, as calculated from their record-breaking aqueous iodine adsorption capacities. Temperature-gated DMSO sorption investigations illustrated that the flexible nature of cross-linkers in HCOFs provides positive entropy from the coexistence of multiple conformations to allow for elastic expansion and contraction of the frameworks.

Variation of topologies and entanglements in metal-organic frameworks with mixed tris[4-(1H-imidazol-1-yl)phenyl]phosphine oxide and dicarboxylate ligands.

Six metal-organic frameworks with the combination of a novel tripodal ligand tris[4-(1H-imidazol-1-yl)phenyl]phosphine oxide (tipo) and different polycarboxylates, namely [Zn2(bpdc)2(tipo)(H2O)]·5H2O (1), [Cd(tipo)2][Cd2(bdc)3]·4H2O (2), [Cd2(ndc)2(tipo)2]·3H2O (3), [Cd2(bpedc)2(tipo)2]·15H2O (4), [Cd2(hfdc)2(tipo)2]·10H2O (5) and [Ni2(tipo)2(bpedc)(H2O)4]·2NO3·2DMF (6), were synthesized and structurally characterized. Compound 1 featured a 2D → 3D parallel polycatenation of a 3,3,4-connected net with an unusual 4 degrees of catenation and an index of separation of 2. Compound 2 showed an unprecedented polycatenation of charged networks 2D kgd and 3D pcu constructed by different organic ligands. Compound 3 exhibited a combination of a 2D → 3D parallel polycatenation of the sql net and polythreading. Compound 4 featured a 4-fold interpenetration of a 3,5-connected 3,5T1 net. Compound 5 displayed an unusual 2-fold interpenetration of the sdd net. Compound 6 presented a 2-fold interpenetration of the 3,4-connected bey net with a cationic framework. The luminescence properties of 1-5 in the solid state were investigated. Also, compound 6 showed a tendency for rapid selective dye capture and highly efficient Cr2O72- removal from aqueous solution.

Topologically guided tuning of Zr-MOF pore structures for highly selective separation of C6 alkane isomers.

As an alternative technology to energy intensive distillations, adsorptive separation by porous solids offers lower energy cost and higher efficiency. Herein we report a topology-directed design and synthesis of a series of Zr-based metal-organic frameworks with optimized pore structure for efficient separation of C6 alkane isomers, a critical step in the petroleum refining process to produce gasoline with high octane rating. Zr6O4(OH)4(bptc)3 adsorbs a large amount of n-hexane but excluding branched isomers. The n-hexane uptake is ~70% higher than that of a benchmark adsorbent, zeolite-5A. A derivative structure, Zr6O4(OH)8(H2O)4(abtc)2, is capable of discriminating all three C6 isomers and yielding a high separation factor for 3-methylpentane over 2,3-dimethylbutane. This property is critical for producing gasoline with further improved quality. Multicomponent breakthrough experiments provide a quantitative measure of the capability of these materials for separation of C6 alkane isomers. A detailed structural analysis reveals the unique topology, connectivity and relationship of these compounds.

Distinguishing Metal-Organic Frameworks.

We consider two metal-organic frameworks as identical if they share the same bond network respecting the atom types. An algorithm is presented that decides whether two metal-organic frameworks are the same. It is based on distinguishing structures by comparing a set of descriptors that is obtained from the bond network. We demonstrate our algorithm by analyzing the CoRe MOF database of DFT optimized structures with DDEC partial atomic charges using the program package ToposPro.

Topological Motifs in Cyanometallates: From Building Units to Three-Periodic Frameworks.

This review focuses on topological features of three-periodic (framework) p, d, and f metal cyano complexes or cyanometallates, i.e. coordination compounds, where CN(-) ligands play the main structure-forming role. In addition, molecular, one-periodic (chain), and two-periodic (layer) cyanometallates are considered as possible building blocks of the three-periodic cyanometallates. All cyanometallates are treated as systems of nodes (mononuclear, polynuclear, or transitional metal cluster complexes) joined together via CN-containing spacers. The most typical nodes and spacers as well as methods of their connection are described and systematized. Particular attention is paid to the overall structural motifs in the three-periodic cyanometallates, especially to the relations between the local coordination (coordination figure) of structural units and the entire framework topology. The chemical factors are discussed that influence the cyanometallate topological properties due to modification of nodes, spacers, or coordination figures.