Non-stackable molecules assemble into porous crystals displaying concerted cavity-changing motions

With small molecules, it is not easy to create large void spaces. Flat aromatics stack tightly, while flexible chains fold to fill the cavities. As an intuitive design to make open channels inside molecularly constructed solids, we employed propeller-shaped bicyclic triazoles to prepare a series of aromatic-rich three-dimensional (3D) building blocks. This modular approach has no previous example, but is readily applicable to build linear, bent, and branched arrays of non-stackable architectural motifs from existing flat aromatics by single-pot reactions. A letter H-shaped molecule thus prepared self-assembles into porous crystals, the highly unusual stepwise gas sorption behaviour of which prompted in-depth studies. A combination of single-crystal and powder X-ray diffraction analysis revealed multiple polymorphs, and sterically allowed pathways for their reversible interconversions that open and close the pores in response to external stimuli.

Like non-collapsible open voids within stacks of steel H-beams, a non-covalent assembly of three-dimensional aromatics produces porous crystals. Concerted motions of the molecular H-beams open and close the cavities in response to external stimuli.

Porous flexible frameworks: origins of flexibility and applications

Porous crystalline frameworks including zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have attracted great research interest in recent years. In addition to their assembly in the solid-state being fundamentally interesting and aesthetically pleasing, their potential applications have now pervaded in different areas of chemistry, biology and materials science. When framework materials are endowed with 'flexibility', they exhibit some properties (e.g., stimuli-induced pore breathing and reversible phase transformations) that are distinct from their rigid counterparts. Benefiting from flexibility and porosity, these framework materials have shown promise in applications that include separation of toxic chemicals, isotopes and hydrocarbons, sensing, and targeted delivery of chemicals. While flexibility in MOFs has been widely appreciated, recent developments of COFs and HOFs have established that flexibility is not just limited to MOFs. In fact, zeolites-that are considered rigid when compared with MOFs-are also known to exhibit dynamic modes. Despite flexibility may be conceived as being detrimental to the formation and stability of periodic structures, the landscape of flexible framework structures continues to expand with discovery of new materials with promising applications. In this review, we make an account of different flexible framework materials based on their framework types with a more focus on recent examples and delve into the origin of flexibility in each case. This systematic analysis of different flexibility types based on their origins enables understanding of structure-property relationships, which should help guide future development of flexible framework materials based on appropriate monomer design and tailoring their properties by bottom-up approach. In essence, this review provides a summary of different flexibility types extant to framework materials and critical analysis of importance of flexibility in emerging applications.

Crystal structure of [3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne]nickel(II) diperchlorate

The square-planar nickel(II) title complex, [Ni(C24H36F2N6)](ClO4)2 or [NiL](ClO4)2 (L = 3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne) was synthesized by a one-pot reaction of template condensation and its X-ray crystal structure was determined. The nickel(II) ion lies close by a twofold axis and the complex displays whole-mol-ecule disorder. Ligand L, a hexa-aza-cyclo-tetra-decane ring having 4-fluoro-phenethyl side chains attached to uncoordinated nitro-gen atoms, adopts a trans III (R,R,S,S) configuration. The average Ni-N bond distance is 1.934 (9) Å, which is quite similar to those of other nickel(II) complexes with similar ligands. The nickel(II) ion is located 0.051 (7) Å above the least-squares plane through the four coordinated N atoms. The average C-N bond distance and C-N-C angle involving uncoordinated nitro-gen atoms are 1.425 (12) Å and 118.0 (9)°, respectively, indicating a significant contribution of sp 2 hybridization for these N atoms. The inter-molecular N-H⋯O, C-H⋯O/F hydrogen bonds of the complex form a network structure, which looks like a seamless floral lace pattern.

Solvent-triggered single-crystal-to-single-crystal transformation from a monomeric to polymeric copper(II) complex based on an aza macrocyclic ligand

Reversible solvent-triggered single-crystal-to-single-crystal (SCSC) transformations are observed between two copper(II) azamacrocyclic complexes: [Cu(C₁₆H₃₈N₆)(H₂O)₂](C₁₂H₆O₄) (1) and [Cu(C₁₆H₃₈N₆)(C₁₂H₆O₄)] (2). Complex (1) was prepared via self-assembly of a copper(II) azamacrocyclic complex containing butyl pendant groups, [Cu(C₁₆H₃₈N₆)(ClO₄)₂], with 2,7-naphthalenedicarboxylic acid. When monomeric compound (1) was immersed in CH₃OH, coordination polymer (2) was obtained, indicating a solvent-triggered SCSC transformation. Furthermore, when (2) was immersed in water, an reverse SCSC transformation from (2) to (1) occurred. Complex (1) presents a 3D supramolecular structure formed via intermolecular hydrogen-bonding interactions, whereas complex (2) features a 1D zigzag coordination polymer. The reversible SCSC transformation of (1) and (2) was characterized using single-crystal X-ray diffraction and in situ powder X-ray diffraction techniques. Despite its poor porosity, complex (2) displayed interesting CO₂ adsorption behaviour under CO₂ gas.

CO2-induced single-crystal to single-crystal transformations of an interpenetrated flexible MOF explained by in situ crystallographic analysis and molecular modeling

A molecular-level investigation is reported on breathing behaviour of a metal-organic framework (1) in response to CO2 gas pressure. High-pressure gas adsorption shows a pronounced step corresponding to a gate-opening phase transformation from a closed (1cp ) to a large-pore (1lp ) form. A plateau is observed upon desorption corresponding to narrow-pore intermediate form 1np which does not occur during adsorption. These events are corroborated by pressure-gradient differential scanning calorimetry and in situ single-crystal X-ray diffraction analysis under controlled CO2 gas pressure. Complete crystallographic characterisation facilitated a rationalisation of each phase transformation in the series 1cp → 1lp → 1np → 1cp during adsorption and subsequent desorption. Metropolis grand-canonical Monte Carlo simulations and DFT-PBE-D3 interaction energy calculations strongly underpin this first detailed structural investigation of an intermediate phase encountered upon desorption.

A Hydrogen-Bonded Organic Framework (HOF) with Type IV NH3 Adsorption Behavior

An S-shaped gas isotherm pattern displays high working capacity in pressure-swing adsorption cycle, as established for CO₂ , CH₄ , acetylene, and CO. However, to our knowledge, this type of adsorption behavior has not been revealed for NH₃ gas. Herein, we design and characterize a hydrogen-bonded organic framework (HOF) that can adsorb NH₃ uniquely in an S-shape (type IV) fashion. While conventional porous materials, mostly with type I NH₃ adsorption behavior, require relatively high regeneration temperature, this platform which has significant working capacity is easily regenerated and recyclable at room temperature.