Recent advances in the structural design and regulation of lanthanide clusters: Formation and self-assembly mechanisms

Abstract

The design and synthesis of lanthanide clusters with novel topological connections and desirable properties are attracting extensive attention. Researchers have obtained numerous types of lanthanide clusters (e.g., tubular, cage, hamburger, and wheel) and examined their magnetocaloric effect, proton conductivity, sensing ability, applicability to lighting, and molecular magnetism. Despite substantial progress, lanthanide clusters are still primarily designed and synthesized via two common strategies: the ligand-controlled hydrolysis method and the anion-template method supported by carboxylic acid ligands. A detailed analysis of the formation and assembly mechanism would enable precisely controlled synthesis and the rapid development of crystal engineering. However, tracking the assembly process is difficult because both common strategies involve complex reactions with unstable intermediate and final products. Therefore, new guiding strategies for the design and construction of lanthanide clusters are urgently demanded. Bulky multidentate chelating ligands can quickly capture Ln(III) ions in solution, forming relatively stable single-template primitives. In the presence of an anion or specific hydrolyzate, single-template primitives assemble into multicomponent template primitives and then into complex lanthanide clusters (LCs). The intermediate template primitives are stable because the outer chelating ligands tightly wrap the inner metal center; consequently, the obtained LCs are highly stable in solution. These stable template primitives enable the identification of intermediates in the self-assembly process and facilitate the rational design and directed construction of LCs. In addition, the rapid capture of Ln(III) ions by multidentate chelating ligands greatly simplifies the self-assembly process, avoiding the formation of many low-stability, difficult-to-discriminate, chaotic, extremely complex, and diverse Ln(III)-ion hydrolysis intermediates and thereby promoting in-depth analysis of the self-assembly mechanisms (SAMs) of LCs. In recent years, multidentate chelate coordination (MCC) has emerged as a promising method for constructing serial LCs with decipherable SAMs. This review highlights the research progress on SAMs of LCs, focusing on the different SAMs of the hydrolysis method, anion-template method, and MCC method. The review highlights that MCC can advance SAM studies of LCs.