A Sodium Germanosilicide with Unusual Network Topology.

The germanosilicide Na4-xGeySi16-y (0.4 ≤ x ≤ 1.1, 4.7 ≤ y ≤ 9.3) was synthesized under high-pressure, high-temperature conditions. The novel guest-host compound comprises a unique tetrel framework with dual channels housing sodium and smaller, empty (Si,Ge)9 units. The arrangement represents a new structure type with an overall structural topology that is closely related to a hypothetical carbon allotrope. Topological analysis of the structure revealed that the guest environment space cannot be tiled with singular polyhedra as in cage compounds (e.g., clathrates). The analysis of natural tilings provides a convenient method to unambiguously compare related tetrel-rich structures and can help elucidate new possible structural arrangements of intermetallic compounds.

An Octacarboxylate-Linked Sodium Metal-Organic Framework with High Porosity.

Alkali metal-based metal-organic frameworks (MOFs) with permanent porosity are scarce because of their high tendency to coordinate with solvents such as water. However, these MOFs are lightweight and bear gravimetric benefits for gas adsorption related applications. In this study, we present the successful construction of a microporous MOF, designated as HIAM-111, built solely on sodium ions by using an octacarboxylate linker. The structure of HIAM-111 is based on 8-connected Na4 clusters and exhibits a novel topology with an underlying 32,42,8-c net. Remarkably, HAM-111 possesses a robust and highly porous framework with a BET surface area of 1561 m2/g, significantly surpassing that of the previously reported Na-MOFs. Further investigations demonstrate that HIAM-111 is capable of separating C2H2/CO2 and purifying C2H4 directly from C2H4/C2H2/C2H6 with high adsorption capacities. The current work may shed light on the rational design of robust and porous MOFs based on alkali metals.

Rational Design and Reticulation of Infinite qbe Rod Secondary Building Units into Metal-Organic Frameworks through a Global Desymmetrization Approach for Inverse C3 H8 /C3 H6 Separation.

The development of reticular chemistry has enabled the construction of a large array of metal-organic frameworks (MOFs) with diverse net topologies and functions. However, dominating this class of materials are those built from discrete/finite secondary building units (SBUs), yet the designed synthesis of frameworks involving infinite rod-shaped SBUs remain underdeveloped. Here, by virtue of a global linker desymmetrization approach, we successfully targeted a novel Cu-MOF (Cu-ASY) incorporating infinite Cu-carboxylate rod SBUs with its structure determined by micro electron diffraction (MicroED) crystallography. Interestingly, the rod SBU can be simplified as a unique cylindric sphere packing qbe tubule made of [43 .62 ] tiles, which further connect the tritopic linkers to give a newly discovered 3,5-connected gfc net. Cu-ASY is a permanent ultramicroporous material featuring 1D channels with highly inert surfaces and shows a preferential adsorption of propane (C3 H8 ) over propene (C3 H6 ). The efficiency of C3 H8 selective Cu-ASY is validated by multicycle breakthrough experiments, giving C3 H6 productivity of 2.2 L/kg. Density functional theory (DFT) calculations reveal that C3 H8 molecules form multiple C-H⋅⋅⋅π and atypical C-H⋅⋅⋅ H-C van der Waals interactions with the inner nonpolar surfaces. This work therefore highlights the linker desymmetrization as an encouraging and intriguing strategy for achieving unique MOF structures and properties.

Unlocking New Topologies in Zr-Based Metal-Organic Frameworks by Combining Linker Flexibility and Building Block Disorder.

The outstanding diversity of Zr-based frameworks is inherently linked to the variable coordination geometry of Zr-oxo clusters and the conformational flexibility of the linker, both of which allow for different framework topologies based on the same linker-cluster combination. In addition, intrinsic structural disorder provides a largely unexplored handle to further expand the accessibility of novel metal-organic framework (MOF) structures that can be formed. In this work, we report the concomitant synthesis of three topologically different MOFs based on the same M6O4(OH)4 clusters (M = Zr or Hf) and methane-tetrakis(p-biphenyl-carboxylate) (MTBC) linkers. Two novel structural models are presented based on single-crystal diffraction analysis, namely, cubic c-(4,12)MTBC-M6 and trigonal tr-(4,12)MTBC-M6, which comprise 12-coordinated clusters and 4-coordinated tetrahedral linkers. Notably, the cubic phase features a new architecture based on orientational cluster disorder, which is essential for its formation and has been analyzed by a combination of average structure refinements and diffuse scattering analysis from both powder and single-crystal X-ray diffraction data. The trigonal phase also features structure disorder, although involving both linkers and secondary building units. In both phases, remarkable geometrical distortion of the MTBC linkers illustrates how linker flexibility is also essential for their formation and expands the range of achievable topologies in Zr-based MOFs and its analogues.

Predicting crystal growth via a unified kinetic three-dimensional partition model.

Understanding and predicting crystal growth is fundamental to the control of functionality in modern materials. Despite investigations for more than one hundred years, it is only recently that the molecular intricacies of these processes have been revealed by scanning probe microscopy. To organize and understand this large amount of new information, new rules for crystal growth need to be developed and tested. However, because of the complexity and variety of different crystal systems, attempts to understand crystal growth in detail have so far relied on developing models that are usually applicable to only one system. Such models cannot be used to achieve the wide scope of understanding that is required to create a unified model across crystal types and crystal structures. Here we describe a general approach to understanding and, in theory, predicting the growth of a wide range of crystal types, including the incorporation of defect structures, by simultaneous molecular-scale simulation of crystal habit and surface topology using a unified kinetic three-dimensional partition model. This entails dividing the structure into 'natural tiles' or Voronoi polyhedra that are metastable and, consequently, temporally persistent. As such, these units are then suitable for re-construction of the crystal via a Monte Carlo algorithm. We demonstrate our approach by predicting the crystal growth of a diverse set of crystal types, including zeolites, metal-organic frameworks, calcite, urea and l-cystine.

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.

Customized Synthesis: Solvent- and Acid-Assisted Topology Evolution in Zirconium-Tetracarboxylate Frameworks.

Metal-organic frameworks (MOFs) demonstrate strong potential for various important applications due to their well tunable structures and compositions through metal and organic linker engineering. As an effective approach, topology evolution by controlling linker conformation has received considerable attention, where solvents and acids have crucial effects on structural formation. However, a systematic study of such effects remains under investigated. Herein, we carried out a methodical study on the topology evolution in Zr-MOFs directed by solvothermal conditions with various combinations of three common solvents and six different acids. As a result, three Zr-MOFs with different topologies, scu (HIAM-4007), scp (HIAM-4008), and csq (HIAM-4009), were obtained using the same Zr6-cluster and tetratopic carboxylate linker, in which structure diversity shows significant influence on their corresponding photoluminescence quantum yields. Further experiments revealed that the acidity of acids and the basicity of solvents strongly influenced the linker conformation in the resultant MOFs, leading to the topology evolution. Such a solvent- and acid-assisted topology evolution represents a general approach that can be used with other tetratopic carboxylate linkers to realize structural diversity. The present work demonstrates an effective structure designing strategy by controlling synthetic conditions, which may prove to be powerful for customized synthesis of MOFs with specific structure and functionality.

The Microscopic Diamond Anvil Cell: Stabilization of Superhard, Superconducting Carbon Allotropes at Ambient Pressure.

A metallic, covalently bonded carbon allotrope is predicted via first principles calculations. It is composed of an sp3 carbon framework that acts as a diamond anvil cell by constraining the distance between parallel cis-polyacetylene chains. The distance between these sp2 carbon atoms renders the phase metallic, and yields two well-nested nearly parallel bands that cross the Fermi level. Calculations show this phase is a conventional superconductor, with the motions of the sp2 carbons being key contributors to the electron-phonon coupling. The sp3 carbon atoms impart superior mechanical properties, with a predicted Vickers hardness of 48 GPa. This phase, metastable at ambient conditions, could be made by on-surface polymerization of graphene nanoribbons, followed by pressurization of the resulting 2D sheets. A family of multifunctional materials with tunable superconducting and mechanical properties could be derived from this phase by varying the sp2 versus sp3 carbon content, and by doping.

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.

Generating carbon schwarzites via zeolite-templating.

Zeolite-templated carbons (ZTCs) comprise a relatively recent material class synthesized via the chemical vapor deposition of a carbon-containing precursor on a zeolite template, followed by the removal of the template. We have developed a theoretical framework to generate a ZTC model from any given zeolite structure, which we show can successfully predict the structure of known ZTCs. We use our method to generate a library of ZTCs from all known zeolites, to establish criteria for which zeolites can produce experimentally accessible ZTCs, and to identify over 10 ZTCs that have never before been synthesized. We show that ZTCs partition space into two disjoint labyrinths that can be described by a pair of interpenetrating nets. Since such a pair of nets also describes a triply periodic minimal surface (TPMS), our results establish the relationship between ZTCs and schwarzites-carbon materials with negative Gaussian curvature that resemble TPMSs-linking the research topics and demonstrating that schwarzites should no longer be thought of as purely hypothetical materials.

The Different Story of π Bonds.

We revisit "classical" issues in multiply bonded systems between main groups elements, namely the structural distortions that may occur at the multiple bonds and that lead, e.g., to trans-bent and bond-length alternated structures. The focus is on the role that orbital hybridization and electron correlation play in this context, here analyzed with the help of simple models for σ- and π-bonds, numerically exact solutions of Hubbard Hamiltonians and first principles (density functional theory) investigations of an extended set of systems.

High-Throughput Electron Diffraction Reveals a Hidden Novel Metal-Organic Framework for Electrocatalysis.

Metal-organic frameworks (MOFs) are known for their versatile combination of inorganic building units and organic linkers, which offers immense opportunities in a wide range of applications. However, many MOFs are typically synthesized as multiphasic polycrystalline powders, which are challenging for studies by X-ray diffraction. Therefore, developing new structural characterization techniques is highly desired in order to accelerate discoveries of new materials. Here, we report a high-throughput approach for structural analysis of MOF nano- and sub-microcrystals by three-dimensional electron diffraction (3DED). A new zeolitic-imidazolate framework (ZIF), denoted ZIF-EC1, was first discovered in a trace amount during the study of a known ZIF-CO3 -1 material by 3DED. The structures of both ZIFs were solved and refined using 3DED data. ZIF-EC1 has a dense 3D framework structure, which is built by linking mono- and bi-nuclear Zn clusters and 2-methylimidazolates (mIm- ). With a composition of Zn3 (mIm)5 (OH), ZIF-EC1 exhibits high N and Zn densities. We show that the N-doped carbon material derived from ZIF-EC1 is a promising electrocatalyst for oxygen reduction reaction (ORR). The discovery of this new MOF and its conversion to an efficient electrocatalyst highlights the power of 3DED in developing new materials and their applications.

A Porous Covalent Organic Framework with Voided Square Grid Topology for Atmospheric Water Harvesting.

Atmospheric moisture is a ubiquitous water resource available at any time and any place, making it attractive to develop materials for harvesting water from air to address the imminent water shortage crisis. In this context, we have been exploring the applicability of covalent organic frameworks (COFs) for water harvesting and report here a new porous, two-dimensional imine-linked COF with a voided square grid topology, termed COF-432. Unlike other reported COFs, COF-432 meets the requirements desired for water harvesting from air in that it exhibits an S-shaped water sorption isotherm with a steep pore-filling step at low relative humidity and without hysteretic behavior-properties essential for energy-efficient uptake and release of water. Further, it can be regenerated at ultra-low temperatures and displays exceptional hydrolytic stability, as demonstrated by the retention of its working capacity after 300 water adsorption-desorption cycles.

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.

Size-Selective Urea-Containing Metal-Organic Frameworks as Receptors for Anions.

Anion recognition by neutral hosts that function in aqueous solution is an emerging area of interest in supramolecular chemistry. The design of neutral architectures for anion recognition still remains a challenge. Among neutral anion receptor systems, urea and its derivatives are considered as "privileged groups" in supramolecular anion recognition, since they have two proximate polarized N-H bonds exploitable for anion recognition. Despite promising advancements in urea-based structures, the strong hydrogen bond drives detrimental self-association. Therefore, immobilizing urea fragments onto the rigid structures of a metal-organic framework (MOF) would prevent this self-association and promote hydrogen-bond-accepting substrate recognition. With this aim, we have synthesized two new urea-containing metal-organic frameworks, namely [Zn(bpdc)(L2)]n·nDMF (TMU-67) and [Zn2(bdc)2(L2)2]n·2nDMF (TMU-68) (bpdc = biphenyl-4,4'-dicarboxylate; bdc = terephthalate; L2 = 1,3-bis(pyridin-4-yl)urea), and we have assessed their recognition ability toward different anions in water. The two MOFs show good water stability and anion affinity, with a particular selectivity toward dihydrogen arsenate for TMU-67 and toward fluoride for TMU-68. Crystal structure characterizations reveal 3-fold and 2-fold interpenetrated 3D networks for TMU-67 and TMU-68, respectively, where all single interpenetrated networks are hydrogen bonded to each other in both cases. Despite the absence of self-quenching, the N-H urea bonds are tightly hydrogen bonded to the oxygen atoms of the dicarboxylate ligands and cannot be directly involved in the recognition process. The good performance in anion sensing and selectivity of the two MOFs can be ascribed to the network interpenetration that, shaping the void, creates monodimensional channels, decorated by exposed oxygen atom sites selective for arsenate sensing in TMU-67 and isolated cavities, covered by phenyl groups selective for fluoride recognition in TMU-68.

Hierarchically Structured Allotropes of Phosphorus from Data-Driven Exploration.

The discovery of materials is increasingly guided by quantum-mechanical crystal-structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data-driven approaches can vastly accelerate the search for complex structures, combining a machine-learning (ML) model for the potential-energy surface with efficient, fragment-based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic ("random") structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one-dimensional (1D) single and double helix structures, nanowires, and two-dimensional (2D) phosphorene allotropes with square-lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures.

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.

Topology of Intermetallic Structures: From Statistics to Rational Design.

More than 38 000 substances made only of metal atoms are collected in modern structural databases; we may call them intermetallic compounds. They have important industrial applications, and yet they are terra incognita for most of our undergraduate students. Their structural complexity and synthesis are not easily adaptable to first years laboratories, keeping them away from the standard curricula. They have been described over the years following alternative and complementary views such as coordination polyhedra, atomic layers, and polyatomic clusters. All of these descriptions, albeit relying on grounded principles, have been applied on a subjective basis and never implemented as a strict computational algorithm. Sometimes, the authors generated multiple views of the same structure reported with beautifully drawn figures and/or photos of hand-crafted models in seminal works of the precomputer age. With the use of our multipurpose crystallochemical program package ToposPro, we explored the structural chemistry of intermetallics with objective and reproducible topological methods that allow us to reconcile different structure descriptions. After computing the connectivity patterns between the metal atoms on the basis of Voronoi partitioning of the crystal space, we were able to group the 38 000 intermetallic compounds into 3700 sets of crystal structures with the same topology of atomic net. We have described the different views used in the literature and shown that 12-vertex polyhedra are the most frequent (33%) and that almost half of them are icosahedron-like (46%), followed by cuboctahedron (25%) and, unexpectedly, by bicapped pentagonal prism (13%). Looking for layers, we have found that the hexagonal lattice, which corresponds to the closest packing of spheres on a plane, exists in more than 11 000 crystal structures, confirming the close-packed nature of intermetallics. We have also applied the nanocluster approach, which goes beyond the first coordination sphere and looks for structural units as multishell clusters that assemble the whole structure. This approach shows that 41% of intermetallics can be assembled with a single nanocluster and that 22.4% of these are packed according to the face-centered cubic motif of the closest packing of spheres in three-dimensional space. We have shown that our approach can easily adopt any other building model and hence could become a platform for a universal predictive scheme. Within this scheme, all of the structural descriptors can be related to experimental data and theoretical modeling results and then can be used to synthesize new intermetallic compounds and to foresee novel materials.

Autoluminescent Metal-Organic Frameworks (MOFs): Self-Photoemission of a Highly Stable Thorium MOF.

A novel thorium(IV) metal-organic framework (MOF), Th(2,6-naphtalenedicarboxylate)2, has been synthesized via solvothermal reaction of thorium nitrate and 2,6-naphtalendicarboxilyc acid. This compound shows a new structural arrangement with an interesting topology and an excellent thermal resistance, as the framework is stable in air up to 450 °C. Most notably, this MOF, combining the radioactivity of its metal center and the scintillation property of the ligand, has been proven capable of spontaneous photon emission.

Data-driven learning and prediction of inorganic crystal structures.

Crystal structure prediction algorithms, including ab initio random structure searching (AIRSS), are intrinsically limited by the huge computational cost of the underlying quantum-mechanical methods. We have recently shown that a novel class of machine learning (ML) based interatomic potentials can provide a way out: by performing a high-dimensional fit to the ab initio energy landscape, these potentials reach comparable accuracy but are orders of magnitude faster. In this paper, we develop our approach, dubbed Gaussian approximation potential-based random structure searching (GAP-RSS), towards a more general tool for exploring configuration spaces and predicting structures. We present a GAP-RSS interatomic potential model for elemental phosphorus, which identifies and correctly "learns" the orthorhombic black phosphorus (A17) structure without prior knowledge of any crystalline allotropes. Using the tubular structure of fibrous phosphorus as an example, we then discuss the limits of free searching, and discuss a possible way forward that combines a recently proposed fragment analysis with GAP-RSS. Examples of possible tubular (1D) and extended (3D) hypothetical allotropes of phosphorus as found by GAP-RSS are discussed. We believe that in the future, ML potentials could become versatile and routine computational tools for materials discovery and design.