Supramolecular Coordination Cages Based on N-Heterocyclic Carbene-Gold(I) Ligands and Their Precursors: Self-Assembly, Structural Transformation and Guest-Binding Properties

Synthesis of an Adaptable Molecular Barrel and Guest Mediated Stabilization of Its Metastable Higher Homologue

Structural and functional modulation of three-dimensional artificial macromolecular systems is of immense importance. Designing supramolecular cages that can show stimuli mediated reversible switching between higher-order structures is quite challenging. We report here construction of a Pd₆ trifacial barrel (1) by coordination self-assembly. Surprisingly, barrel 1 was found to exhibit guest-responsive behavior. In presence of fullerenes C₆₀ and C₇₀, 1 unprecedentedly transformed to its metastable higher homologue Pd₈ tetrafacial barrel (2), forming stable host-guest complexes (C₆₀)₃⊂2 and (C₇₀)₂⊂2, respectively. Again, encapsulated fullerenes could be extracted from the cavity of 2 using 1,2-dichlorobenzene, leading to its facile conversion to the parent trifacial barrel 1. Such reversible structural interconversion between an adaptable molecular barrel and its guest stabilized higher homologue is an uncommon observation.

Zirconium Metal-Organic Cages: Synthesis and Applications

ConspectusFor the last two decades, materials scientists have contributed to a growing library of porous crystalline materials. These synthetic materials are typically extended networks, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), or discrete materials like metal-organic cages (MOCs) and porous organic cages (POCs). Advanced porous materials have shown promise for various applications due to their modular nature and structural tunability. MOCs have recently garnered attention because of their molecularity that bestows them with many unique possibilities (e.g., solution-processability, structural diversity, and postsynthetic processability).MOCs are discrete molecular assemblies of organic ligands coordinated with either metal cations or metal oxide clusters of different nuclearities, resulting in architectures with inherent porosity. Notably, the molecular nature of MOCs endows them with easy solution-processability unattainable with traditional framework materials. To date, a number of stable MOCs have been reported, such as those based on Rh (Rh-O bond energy: 405 ± 42 kJ/mol), Fe (Fe-O bond energy: 407.0 ± 1.0 kJ/mol), Cr (Cr-O bond energy: 461 ± 8.7 kJ/mol), Ti (Ti-O bond energy: 666.5 ± 5.6 kJ/mol), and Zr (Zr-O bond energy: 766.1 ± 10.6 kJ/mol). Paddle-wheel MOCs have also shown great stability in aqueous environments due to their rigid backbones. The zirconium MOC (Zr-MOCs) family emerges as a class of very robust cages for which their high bond energy endows them with high hydrothermal stability.In 2013, we reported the first four zirconocene tetrahedrons assembled from trinuclear zirconium oxide clusters with ditopic or tritopic organic ligands. Since then, significant progress in the rational design of Zr-MOC has led to an assortment of structures dedicated to meaningful applications.In this Account, we highlight the recent progress in synthesizing Zr-MOCs and Zr-MOC-based higher dimensional frameworks and their applications dedicated in our laboratories and beyond. The general Zr-MOC synthetic strategy involves assembling Zr trinuclear clusters with organic ligands (rigid or flexible) containing various functional groups. This chemistry has afforded cages with structural versatility and active sites, e.g., amino groups, for postsynthetic modifications (PSMs). Since the extrinsic porosity of cage-based frameworks is relatively weak, the resulting frameworks are susceptible to structural rearrangement after solvent removal. To circumvent this limitation, increasing the hydrogen bond ratio and strength between interlinked cages and conducting in situ catalytic polymerizations have been reported to afford permanently porous structures amenable to host-guest reactions.To expand their potential applications, multifunctional Zr-MOCs are highly desired. Such multivariate MOCs can be attained by either employing the isoreticular expansion strategy to create MOCs with high surface areas or using mixed-ligand approaches to afford heterogeneous MOCs. In addition, amorphous MOCs, flexible organic ligands, new functionalities, and MOC-based extended networks are exciting new approaches to developing materials with structural versatility and enhanced characteristics. Thereby, we believe the stability and versatility of the Zr-MOC family hold great potential in expanding and addressing challenging applications.

N-Heterocyclic carbenes and their precursors in functionalised porous materials

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.