Coordination Chemistry in the Structural and Functional Exploration of Actinide-Based Metal-Organic Frameworks

Abstract

■■ 1. Introduction Coordination chemistry is one of the major thrusts of inorganic chemistry pertaining to compounds bearing coordination bonds between metal centers and ligands. In a coordination compound, the pair of shared electrons that forms the coordination bond is solely provided by the ligand. The concepts of coordination geometry and coordination number, established by Werner in the 1890s, have been ceaselessly guiding the development of this field. 1 In particular, these concepts have been invaluable to the burgeoning field of metal-organic frameworks (MOFs), which lies at the intersection of inorganic coordination chemistry and organic chemistry. 2 MOFs are porous, crystalline materials composed of inorganic metal ion/cluster-based nodes and organic linkers; the high tunability of both building blocks has offered vast opportunities for structural and functional explorations of MOFs in applications such as gas storage/ separation, 3 catalysis, 4 enzyme encapsulation, 5 and sensors 6. Fig. 1 Examples of actinide-based inorganic nodes for the construction of MOFs. While transition metal coordination chemistry has been extensively investigated, the chemistry of actinide elements, which is essential for mitigating nuclear waste, 7 has been largely elusive. 8 The actinides reside in the 5f block of the periodic table; uranium and thorium specifically are the most widely explored of the actinides due to the relatively high abundance of their low radioactivity isotopes. 9 While actinide coordination chemistry shares some aspects with transition metal coordination chemistry, actinides have certain unique characteristics that have given rise to unusual and intriguing structure, reactivity, and physical properties. 10 The past few decades have witnessed the emergence of MOFs, which marry inorganic chemistry to organic chemistry through the coordination bonds between metal ion/cluster-based nodes and organic linkers. As compared to transition The coordination chemistry between inorganic and organic species can be optimally exemplified by metal-organic frameworks (MOFs), whose structures and functionalities can be rationally designed from these highly tunable building blocks. The high porosity, stability, and versatile functionalities of MOFs have attracted widespread attention from energy-related research and pollution remediation to biomedical applications. A unique and underexplored subset of these materials are MOFs based on actinide nodes; these MOFs have distinguished themselves as a unique platform for investigating the versatile oxidation states, reactivity, and coordination chemistry of actinides. Herein, we will focus on the rational design and synthesis of actinide-based MOFs under the general guidelines of coordination chemistry for their structural and functional explorations. The dimensionality, topology, and structures of actinide-based MOFs can be controlled by selecting pre-designed building blocks of actinide-based nodes and organic linkers with certain desired coordination geometries and functionalities. These unique actinide-based MOFs have shown promise for applications in nuclear waste mitigation, pollution control, and catalysis.