Marc Schmidtmann

Modular and predictable assembly of porous organic molecular crystals

Nanoporous molecular frameworks are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores rather than, for example, the functional group localization found in the reactive sites of enzymes. This is a potential limitation for ‘one-pot’ chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores. In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally, allowing in silico materials design strategies. The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules.

Archimedean Synthesis and Magic Numbers: “Sizing” Giant Molybdenum-Oxide-Based Molecular Spheres of the Keplerate Type

Pythagorean harmony can be found in the spherical polyoxometalate clusters described here (see illustration for an example of a structure), since there are interesting relationships between the so-called magic numbers (12, 32, 42, 72, 132) relevant for spherical viruses and the number of the building blocks in the cluster. The size of these Keplerate clusters can be tailored by varying the type of connections between the pentagons by means of different spacers.

Organizational Forms of Matter: An Inorganic Super Fullerene and Keplerate Based on Molybdenum Oxide

Plato and Kepler would have been pleased. Despite the large number of atoms present the cluster anion 1 resembles an icosahedral-type structure. This represents definitively an unprecedented event in chemistry! The structure is made up of 12 {Mo11 } fragments such that the fivefold symmetry axes are retained in the resulting spherical object. As an inscribed icosahedron can be recognized in the spherical shell of 1 (see picture), similarities with Keplers famous shell model of the cosmos can be seen.

Materielle Organisationsformen: ein anorganisches Superfulleren und Keplerat auf Molybdänsauerstoffbasis

Da hatten wohl auch Platon und Kepler ihre Freude gehabt! Trotz der grosen Zahl an Atomen weist das Cluster-Anion 1 eine ikosaedrische Struktur auf – sicherlich ein Novum in der Geschichte der Chemie! Es setzt sich derart aus 12 {Mo11}-Fragmenten zusammen, das im resultierenden kugelformigen Gebilde die funfzahligen Drehachsen erhalten bleiben. Da man in 1 ein in eine Kugel einbeschriebenes Ikosaeder erkennen kann, ahnelt dieses Anion strukturell dem beruhmten Keplerschen Schalenmodell des Kosmos (siehe Bild).

Molecular Doping of Porous Organic Cages

Porous organic cages can act as hosts for the three-dimensional alignment of guests such as halogens and organometallics. Porous single crystals are doped by vapor sublimation to produce diamondoid arrangements of guests such as I(5)(-) and OsO(4), leading to marked conductivity enhancement in the case of I(5)(-).

On crystal versus fiber Formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.

Open and shut for guests in molybdenum-oxide-based giant spheres, baskets, and rings containing the pentagon as a common structural element

A novel exchange between ligands and/or guest molecules can be accomplished in giant molecular spheres (an example is shown in the picture) which are in equilibrium with the corresponding giant baskets in solution.

Large Self‐Assembled Chiral Organic Cages: Synthesis, Structure, and Shape Persistence

Keep the cage filled: Two large organic cages (see example) with void diameters of 1.2 nm were synthesized through [8+12] imine condensation reactions. The materials become amorphous upon solvent removal and show little permanent porosity. Molecular dynamics simulations give an insight into the mechanism of these processes, suggesting strategies for synthesizing larger shape-persistent organic cages in the future.

A New Type of Supramolecular Compound: Molybdenum-Oxide-Based Composites Consisting of Magnetic Nanocapsules with Encapsulated Keggin-Ion Electron Reservoirs Cross-Linked to a Two-Dimensional Network

Nanosized metal-oxide-based composites-novel supramolecular entities-have been assembled and even cross-linked under one-pot conditions. The supramolecular entity (see picture) consists of a paramagnetic icosahedral capsule of the type {Mo VI 72 Fe III 30 } as a host which encloses a potential electron-reservoir noncovalently bonded guest, the reduced Keggin cluster [H 2 PMo 12 O 40 ] 3- .

Reversible water uptake by a stable imine-based porous organic cage

A crystalline porous organic cage molecule, CC3, is shown to adsorb up to 20.1 wt% water reversibly. This was confirmed by both gravimetric sorption and by crystallographic analysis. Crystals of CC3 are stable in boiling water for at least 4 h. The surprising chemical and supramolecular stability of these imine-based molecular crystals suggests scope for practical applications in humid environments.