Molecular Borromean Rings Based on Half-Sandwich Organometallic Rectangles

Over the last two decades, interlocked molecular species have received considerable attention, not only because of their intriguing structures and topological importance, but also because of their potential applications as smart materials, nanoscale devices, and molecular machines. Through judicious choice of metal centers and their adjoining ligands, a range of interesting interlocked structures have been realized by coordination-driven self-assembly. In addition, researchers have extensively developed synthetic methodologies for the construction of organized self-assemblies. One fascinating and challenging synthetic target in this field is the family of molecular Borromean rings, which consist of three chemically independent rings that are locked in such a way that no two of the three rings are linked with each other. Toward this goal, we have developed a template-free self-assembly method for synthesizing molecular Borromean rings by rationally designing metal-containing precursors and organic ligands. In this Account, we present our recent work, focusing on interlocked structures comprising half-sandwich iridium- and rhodium-based organometallic assemblies obtained by rational design. We first describe a series of template-free self-assembled organometallic molecular Borromean rings, which we constructed from preorganized binuclear half-sandwich molecular clips and suitable pyridyl ligands. These molecular Borromean rings can be sorted into four types according to their different bridging ligands, including those based on metallaligands, dihalogenated ligands, naphthazarin and π-acceptor ligands. By single-crystal X-ray crystallographic analysis, NMR experiment, and DFT calculation, we discuss their driving forces and the inter-ring interactions. Furthermore, we took advantage of the dissimilarity in their interactions to realize selective, reversible conversions between molecular Borromean rings and monomeric rectangles by the use of suitable solvents or guest molecules. Subsequently, a stepwise chemoseparation method based on molecular Borromean rings was established, with the molecular Borromean rings used in the separation being recoverable and recyclable. Due to their structural complexity and difficult synthesis, useful guidelines or rules to help design complicated interlocked molecules are highly desirable. We also highlight our efforts to develop empirical guidelines to uncover the relationship between the aspect ratio of metallarectangles and the formation or stability of molecular Borromean rings. An empirical formula has further been established to show the approximate ratio of lengths of the short arm and the long arm in molecular Borromean rings based on π-π (or p-π) stacking. We then demonstrate how to use these guidelines to design new molecular Borromean rings and further lead to other interlocked structures, for example, [2]- and [3]catenane structures. Taken together, our results may lead to a promising future for the design of fascinating and useful interlocked structures by coordination-driven self-assembly.