Porous Metal-Organic Polyhedra: Morphology, Porosity, and Guest Binding

Self-Assembly of Chiral Porous Metal-Organic Polyhedra from Trianglsalen Macrocycles

Metal-organic polyhedra (MOPs) can exhibit tunable porosity and functionality, suggesting potential for applications such as molecular separations. MOPs are typically constructed by the bottom-up multicomponent self-assembly of organic ligands and metal ions, and the final functionality can be hard to program. Here, we used trianglsalen macrocycles as preorganized building blocks to assemble octahedral-shaped MOPs. The resultant MOPs inherit most of the preorganized properties of the macrocyclic ligands, including their well-defined cavities and chirality. As a result, the porosity in the MOPs could be tuned by modifying the structure of the macrocycle building blocks. Using this strategy, we could systematically enlarge the size of the MOPs from 26.3 to 32.1 Å by increasing the macrocycle size. The family of MOPs shows experimental surface areas of up to 820 m2/g, and they are stable in water. One of these MOPs can efficiently separate the rare gases Xe from Kr because the prefabricated macrocyclic windows of MOPs can be modified to sit at the Xe/Kr size cutoff range.

Porous Metal-Organic Cages Based on Rigid Bicyclo[2.2.2]oct-7-ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

Three new ligands containing a bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxydiimide unit have been used to assemble lantern-type metal-organic cages with the general formula [Cu4 L4 ]. Functionalisation of the backbone of the ligands leads to distinct crystal packing motifs between the three cages, as observed with single-crystal X-ray diffraction. The three cages vary in their gas sorption behaviour, and the capacity of the materials for CO2 is found to depend on the activation conditions: softer activation conditions lead to superior uptake, and one of the cages displays the highest BET surface area found for lantern-type cages so far.

Control of evolution of porous copper-based metal-organic materials for electroreduction of CO2 to multi-carbon products

Electrochemcial reduction of CO₂ to multi-carbon (C₂₊) products is an important but challenging task. Here, we report the control of structural evolution of two porous Cu(ii)-based materials (HKUST-1 and CuMOP, MOP = metal-organic polyhedra) under electrochemical conditions by adsorption of 7,7,8,8-tetracyanoquinodimethane (TNCQ) as an additional electron acceptor. The formation of Cu(i) and Cu(0) species during the structural evolution has been confirmed and analysed by powder X-ray diffraction, and by EPR, Raman, XPS, IR and UV-vis spectroscopies. An electrode decorated with evolved TCNQ@CuMOP shows a selectivity of 68% for C₂₊ products with a total current density of 268 mA cm^-2 and faradaic efficiency of 37% for electrochemcial reduction of CO₂ in 1 M aqueous KOH electrolyte at -2.27 V vs. RHE (reversible hydrogen electrode). In situ electron paramagnetic resonance spectroscopy reveals the presence of carbon-centred radicals as key reaction intermediates. This study demonstrates the positive impact of additional electron acceptors on the structural evolution of Cu(ii)-based porous materials to promote the electroreduction of CO₂ to C₂₊ products.

Hydrogen Isotope Separation Using a Metal–Organic Cage Built from Macrocycles

Porous materials that contain ultrafine pore apertures can separate hydrogen isotopes via kinetic quantum sieving (KQS). However, it is challenging to design materials with suitably narrow pores for KQS that also show good adsorption capacities and operate at practical temperatures. Here, we investigate a metal–organic cage (MOC) assembled from organic macrocycles and Zn^II ions that exhibits narrow windows (<3.0 Å). Two polymorphs, referred to as 2α and 2β, were observed. Both polymorphs exhibit D₂/H₂ selectivity in the temperature range 30–100 K. At higher temperature (77 K), the D₂ adsorption capacity of 2β increases to about 2.7 times that of 2α, along with a reasonable D₂/H₂ selectivity. Gas sorption analysis and thermal desorption spectroscopy suggest a gate‐opening effect of the MOCs pore aperture. This promotes KQS at temperatures above liquid nitrogen temperature, indicating that MOCs hold promise for hydrogen isotope separation in real industrial environments.

Metal-organic cages against toxic chemicals and pollutants

The continuous release of toxic chemicals and pollutants into the atmosphere and natural waters threatens, directly and indirectly, human health, the sustainability of the planet, and the future of society. Materials capable of capturing or chemically inactivating hazardous substances, which are harmful to humans and the environment, are critical in the modern age. Metal-organic cages (MOCs) show great promise as materials against harmful agents both in solution and in solid state. This Highlight features examples of MOCs that selectively encapsulate, adsorb, or remove from a medium noxious gases, toxic organophosphorus compounds, water pollutant oxoanions, and some emerging organic contaminants. Remarkably, the toxicity of interacting contaminants may be lowered by MOCs as well. Specific cases pertaining to the use of these cages for the chemical degradation of some harmful substances are presented. This Highlight thus aims to provide an overview of the possibilities of MOCs in this area and new methodological insights into their operation for enhancing their activity and the engineering of further remediation applications.

Assembling metal-organic cages as porous materials

There is growing interest in metal-organic cages (MOCs) as porous materials owing to their processability in solution. The discrete molecular character and surface features of MOCs have a direct impact on the interactions between cages, enabling the final physical state of the materials to be tuned. In this tutorial review, we discuss how to use MOCs as core building units, highlighting the role played by surface functionalisation of MOCs in leading to porous materials in a range of states covering crystalline solids, soft matter, liquids and composites. We finish by providing an outlook on the opportunities for this work to serve as a foundation for the development of increasingly complex functional porous materials structured over various length scales.

A Series of Functionalized Zirconium Metal-Organic Cages for Efficient CO2/N2 Separation

Global warming associated with CO₂ emission has led to frequent extreme weather events in recent years. Carbon capture using porous solid adsorbents is promising for addressing the greenhouse effect. Herein, we report a series of robust metal-organic cages (MOCs) featuring various functional groups, such as methyl and amine groups, for CO₂/N₂ separation. Significantly, the amine-group-functionalized MOC-QW-3-NH₂ displays the best selective CO₂ adsorption performance, as confirmed by single-component adsorption and transient breakthrough experiments. The distinct CO₂ adsorption mechanism has been well studied via theoretical calculations, confirming that the amine groups play a vital role for efficiently selective CO₂ adsorption resulting from hierarchical adsorbate-framework interaction.

Heterogeneous postassembly modification of zirconium metal-organic cages in supramolecular frameworks

We report a heterogeneous postassembly modification (PAM) to synthesize a zirconium metal-organic cage decorated with acrylate functional groups, ZrT-1-AA, which cannot be synthesized by direct coordination-driven self-assembly owing to the reactivity and instability of the ligand. The PAM process is carried out in a single-crystal-to-single-crystal transformation under mild reaction conditions with high efficiency, which is confirmed by ESI-TOF-MS and 1H NMR. In addition, ZrT-1-AA is crosslinked into shaped materials to demonstrate its potential applications. The proposed PAM strategy sheds light on the development of Zr-MOCs decorated with reactive functional groups, whose introduction is challenging or impossible via direct self-assembly.

Gas Storage in Porous Molecular Materials

Molecules with permanent porosity in the solid state have been studied for decades. Porosity in these systems is governed by intrinsic pore space, as in cages or macrocycles, and extrinsic void space, created through loose, intermolecular solid-state packing. The development of permanently porous molecular materials, especially cages with organic or metal-organic composition, has seen increased interest over the past decade, and as such, incredibly high surface areas have been reported for these solids. Despite this, examples of these materials being explored for gas storage applications are relatively limited. This minireview outlines existing molecular systems that have been investigated for gas storage and highlights strategies that have been used to understand adsorption mechanisms in porous molecular materials.