Covalent Post-Synthetic Modification of Metal-Organic Cages: Concepts and Recent Progress

Metal-organic cages (MOCs) are supramolecular coordination complexes that have internal cavities for hosting guest molecules and exhibiting various properties. However, the functions of MOCs are limited by the choice of the building blocks. Post-synthetic modification (PSM) is a technique that can introduce new functional groups and replace existing ones on the MOCs without changing their geometry. Among many PSM methods, covalent PSM is a promising approach to modify MOCs with tailored structures and functions. Covalent PSM can be applied to either the internal cavity or the external surface of the MOCs, depending on the functionality expected to be customized. However, there are still some challenges and limitations in the field of covalent PSM of MOCs, such as the balance between the stability of MOCs and the harshness of organic reactions involved in covalent PSMs. This concept article introduces the organic reaction types involved in covalent PSM of MOCs, their new applications after modification, and summarizes and provides an outlook of this research field.

Porous supramolecular gels produced by reversible self-gelation of ruthenium-based metal-organic polyhedra

Supramolecular gels based on metal-organic polyhedra (MOPs) represent a versatile platform to access processable soft materials with controlled porosity. Herein, we report a self-gelation approach that allows the reversible assembly of a novel Ru-based MOP in the form of colloidal gels. The presence of cationic mixed-valence [Ru2(COO)4]+ paddlewheel units allows for modification of the MOP charge via acid/base treatment, and therefore, its solubility. This feature enables control over supramolecular interactions, making it possible to reversibly force MOP aggregation to form nanoparticles, which further assemble to form a colloidal gel network. The gelation process was thoroughly investigated by time-resolved ζ-potential, pH, and dynamic light scattering measurements. This strategy leads to the evolution of hierarchically porous aerogel from individual MOP molecules without using any additional component. Furthermore, we demonstrate that the simplicity of this method can be exploited for the obtention of MOP-based gels through a one-pot synthetic approach starting from MOP precursors.

Propagating MOF flexibility at the macroscale: the case of MOF-based mechanical actuators

Shapeshifting materials have captured the imagination of researchers for their myriad potential applications, yet their practical development remains challenging. These materials operate by mechanical actuation: their structural responses to external stimuli generate mechanical work. Here, we review progress on the use of flexible metal-organic frameworks (MOFs) in composite actuators that shapeshift in a controlled fashion. We highlight the dynamic behaviour of flexible MOFs, which are unique among materials, even other porous ones, and introduce the concept of propagation, which involves the efficient transmission of flexible MOF deformations to the macroscale. Furthermore, we explain how researchers can observe, measure, and induce such effects in MOF composites. Next, we review pioneering first-generation MOF-composite actuators that shapeshift in response to changes in humidity, temperature, pressure, or to other stimuli. Finally, we allude to recent developments, identify remaining R & D hurdles, and suggest future directions in this field.

Strategies for the Construction of Functional Materials Utilizing Presynthesized Metal-Organic Cages (MOCs)

Metal-organic cages (MOCs) that assemble from metal ions or metal clusters and organic ligands have attracted the interest of the scientific community because of their various functional coordination cavities. Unlike metal-organic frameworks (MOFs) with infinite frameworks, MOCs have discrete structures, making them soluble and stable in certain solvents and facilitating their application as starting reagents in the further construction of single components or composite materials. In recent years, increasing progress has been made in this field. In this review, we introduce these works from the perspective of design strategies, and focus on how presynthesized MOCs can be used to construct functional materials. Finally, we discuss the challenges and development prospects in this field.

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.

Post-synthetic modifications of metal–organic cages

Metal-organic cages (MOCs) are discrete, supramolecular entities that consist of metal nodes and organic linkers, which can offer solution processability and high porosity. Thereby, their predesigned structures can undergo post-synthetic modifications (PSMs) to introduce new functional groups and properties by modifying the linker, metal node, pore or surface environment. This Review explores current PSM strategies used for MOCs, including covalent, coordination and noncovalent methods. The effects of newly introduced functional groups or generated complexes upon the PSMs of MOCs are also detailed, such as improving structural stability or endowing desired functionalities. The development of the aforementioned design principles has enabled systematic approaches for the development and characterization of families of MOCs and, thereby, provides insight into structure-function relationships that will guide future developments.

Metal-Organic Polyhedra as Building Blocks for Porous Extended Networks

Metal-organic polyhedra (MOPs) are a subclass of coordination cages that can adsorb and host species in solution and are permanently porous in solid-state. These characteristics, together with the recent development of their orthogonal surface chemistry and the assembly of more stable cages, have awakened the latent potential of MOPs to be used as building blocks for the synthesis of extended porous networks. This review article focuses on exploring the key developments that make the extension of MOPs possible, highlighting the most remarkable examples of MOP-based soft materials and crystalline extended frameworks. Finally, the article ventures to offer future perspectives on the exploitation of MOPs in fields that still remain ripe toward the use of such unorthodox molecular porous platforms.

Surface chemistry of metal-organic polyhedra

Metal-organic polyhedra (MOPs) are discrete, intrinsically-porous architectures that operate at the molecular regime and, owing to peripheral reactive sites, exhibit rich surface chemistry. Researchers have recently exploited this reactivity through post-synthetic modification (PSM) to generate specialised molecular platforms that may overcome certain limitations of extended porous materials. Indeed, the combination of modular solubility, orthogonal reactive sites, and accessible cavities yields a highly versatile molecular platform for solution to solid-state applications. In this feature article, we discuss representative examples of the PSM chemistry of MOPs, from proof-of-concept studies to practical applications, and highlight future directions for the MOP field.

When polymerization meets coordination-driven self-assembly: metallo-supramolecular polymers based on supramolecular coordination complexes

Polymers have greatly changed and are still changing the way we live ever since, and the construction of novel polymers as functional materials remains an attractive topic in polymer science and related areas. During the past few years, the marriage of discrete supramolecular coordination complexes (SCCs), including two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages, and polymers gave rise to two novel types of metallo-supramolecular polymers, i.e., metallacycle/metallacage-cored star polymers (MSPs) and metallacycle/metallacage-crosslinked polymer networks (MPNs), which has attracted increasing attention and emerged as an exciting new research direction in polymer chemistry. Attributed to their well-defined and diverse topological architectures as well as the unique dynamic features of metallacycles/metallacages as cores or crosslinks, these novel polymers have shown extensive applications. In this review, aiming at providing a practical guide to this emerging area, the introduction of synthetic strategies towards MSPs and MPNs will be presented. In addition, their wide applications in areas such as functional materials, molecular sieving, drug delivery, bacterial killing and bioimaging are also discussed.

A covalent deprotection strategy for assembling supramolecular coordination polymers from metal-organic cages

A Cu₄L₄ metal-organic cage (MOC) composed of amine-protected ligands forms supramolecular coordination polymers (SCPs) upon covalent post-assembly deprotection. The amorphous SCPs form by virtue of aniline-copper coordination and possess a tunable porosity based on the rate of deprotection.