Higher-Order Cu-Based Cages via Subcomponent Self-Assembly.

ConspectusCoordination cages formed via subcomponent self-assembly have found applications in fields including separation, sensing, catalysis, and the stabilization of reactive species, due to their guest binding abilities. Subcomponent self-assembly, which combines dynamic covalent bond (C═N) formation and reversible metal coordination (N→Metal), has enabled the preparation of many intricate polyhedral structures with minimal synthetic effort. This method has been used to prepare multitopic pyridyl-imine ligands that form the edges or faces of polyhedra, with octahedral metal ions, including Fe, Co, and Zn, defining the vertices. The use of Cu in subcomponent self-assembly is less widely reported, as the tetrahedral coordination geometry of Cu requires only two bidentate ligands, which can lead to lower-nuclearity assemblies instead of three-dimensional cages. The coordination flexibility of Cu also adds a challenge to the fabrication of well-defined nanostructures. This Account summarizes a series of higher-order Cu-based coordination cages and the design principles derived from their syntheses. Starting with the development of Cu assemblies and the challenges of preparing Cu cages, we discuss the circumvention of oligomer formation and control of the self-assembly process with Cu through (i) ligand engineering, (ii) vertex design, and (iii) guest-induced structural transformations. Aromatic stacking between corranulene-containing ligands is exploited to produce a 5-fold interlocked [2]catenane, whereas the incorporation of a sterically hindered triptycene subcomponent that minimizes aromatic stacking produces a double-octahedron and a hexagonal prism. These structures illustrate the importance of ligand engineering for obtaining complex Cu structures. We also explored the formation of cages with homo- or heterobimetallic vertices via two distinct strategies. First, dicopper(I) helicates were employed as cage vertices, and second, subcomponents with nonconverging coordination vectors were used. Such bimetallic vertices are challenging to incorporate when octahedral metal templates are used, but the flexibility of Cu renders them accessible. The closed-shell electronic configuration of Cu can endow the cages with photoluminescence, providing circularly polarized luminescence in the presence of helicity-enriched dicopper(I) vertices. The flexible coordination sphere of Cu also facilitates structural transformations upon the addition of suitable guests. One such system is able to self-sort to express the most thermodynamically stable host-guest complex and also undergo structural changes in response to different temperatures and solvents. The insights gained about the structural bases of these Cu cages may help enable the design of other novel Cu nanostructures with functions that may usefully differ from cages that exclusively incorporate octahedral metal centers.