Self-Assembly of Polyoxometalate-Based Sub-1 nm Polyhedral Building Blocks into Rhombic Dodecahedral Superstructures

Self-assembly of subnanometer (sub-1 nm) scale polyhedral building blocks can yield some superstructures with novel and interesting morphology as well as potential functionalities. However, achieving the self-assembly of sub-1 nm polyhedral building blocks is still a great challenge. Herein, through encapsulating the titanium-substituted polyoxometalate (POM, K₇ PTi₂ W₁₀ O₄₀ ) with tetrabutylammonium cations (TBA⁺ ), we first synthesized a sub-1 nm rhombic dodecahedral building block by further tailoring the spatial distribution of TBA⁺ on the POM. Molecular dynamics (MD) simulations demonstrated the eight TBA⁺ cations interacted with the POM cluster and formed the sub-1 nm rhombic dodecahedron. As a result of anisotropy, the sub-1 nm building blocks have self-assembled into rhombic dodecahedral POM (RD-POM) assemblies at the microscale. Benefiting from the regular structure, Br^- ions, and abundant active sites, the obtained RD-POM assemblies exhibit excellent catalytic performance in the cycloaddition of CO₂ with epoxides without co-catalysts. This work provides a promising approach to tailor the symmetry and structure of sub-1 nm building blocks by tuning the spatial distribution of ligands, which may shed light on the fabrication of superstructures with novel properties by self-assembly.