Assembly of silver Trigons into a buckyball-like Ag₁₈₀ nanocage

Buckminsterfullerene (C₆₀) represents a perfect combination of geometry and molecular structural chemistry. It has inspired many creative ideas for building fullerene-like nanopolyhedra. These include other fullerenes, virus capsids, polyhedra based on DNA, and synthetic polynuclear metal clusters and cages. Indeed, the regular organization of large numbers of metal atoms into one highly complex structure remains one of the foremost challenges in supramolecular chemistry. Here we describe the design, synthesis, and characterization of a Ag₁₈₀ nanocage with 180 Ag atoms as 4-valent vertices (V), 360 edges (E), and 182 faces (F)--sixty 3-gons, ninety 4-gons, twelve 5-gons, and twenty 6-gons--in agreement with Euler's rule V - E + F = 2. If each 3-gon (or silver Trigon) were replaced with a carbon atom linked by edges along the 4-gons, the result would be like C₆₀, topologically a truncated icosahedron, an Archimedean solid with icosahedral (I_h) point-group symmetry. If C₆₀ can be described mathematically as a curling up of a 6.6.6 Platonic tiling, the Ag₁₈₀ cage can be described as a curling up of a 3.4.6.4 Archimedean tiling. High-resolution electrospray ionization mass spectrometry reveals that {Ag₃}_n subunits coexist with the Ag₁₈₀ species in the assembly system before the final crystallization of Ag₁₈₀, suggesting that the silver Trigon is the smallest building block in assembly of the final cage. Thus, we assign the underlying growth mechanism of Ag₁₈₀ to the Silver-Trigon Assembly Road (STAR), an assembly path that might be further employed to fabricate larger, elegant silver cages.