Connecting metal–organic cages (MOCs) for CO2 remediation

Abstract

In this Perspective article, recent developments in the self-assembly of supramolecular porous materials made of zero-dimensional (0D) porous metal organic cages (MOCs), connected by different approaches, and their application in CO₂ remediation are reviewed. The connection of MOCs carried out by coordination-bond driven linking, covalent bond linking and mechanical bond formation, leads to the formation of novel smart materials ranging from solid to gel states of matter, and in some cases, porous materials with hierarchical porosity stemming from the intrinsic MOC porosity and the voids generated among the connected MOCs. Both porosities can be tuned depending on the MOC sizes and on the way the cages are connected (i.e., coordination-driven and covalent bond linking). In general, the supramolecular, often networked, materials arising from the connection of MOCs tend to show diffuse scattering, denoting only short-range order, making their structural elucidation very challenging or simply not possible. Thus, the bulk structure of materials formed by connected MOCs is often deduced from information obtained through other techniques like powder XRD, pair distribution function (PDF) analysis, solid-state NMR, dynamic light scattering (DLS), and positron annihilation lifetime spectroscopy (PALS). Materials obtained upon the connection of MOCs are usually in the form of amorphous solids, gels, xerogels and porous liquids (PLs) but have shown to outperform the CO₂ capacity and improved mechanical properties when compared to the single MOCs. The preparation of materials at the interface between solids and liquids is important to create functional materials displaying unique properties such as porosity in liquids and gels arising from the MOCs and their networked assembly, hence allowing CO₂ gas diffusion into the cage's voids where CO₂ can be trapped or catalytically transformed into other molecules (i.e., formic acid (HCOOH)). Thus, materials containing connected MOCs can be exploited where MOFs cannot be used due to their brittle nature, typical of solid crystalline materials, for instance in applications where the porous material has to fit to different shapes for example those required to fit in tubes or shapes that usually crystalline solids cannot adapt easily, thus expanding the applications of connected MOC materials in areas involving fluids.