Assembly of silver Trigons into a buckyball-like Ag ₁₈₀ nanocage

Significance

Here we present a striking outcome from the alliance between chemistry and mathematics in the design, synthesis, and characterization of a silver cage, Ag ₁₈₀. In principle, the design replaces each carbon atom of C ₆₀ with a triplet of argentophilicity-bonded silver atoms to produce a 3.4.6.4 (1,1) polyhedron with sixty 3-gons, ninety 4-gons, twelve 5-gons, and twenty 6-gons. Results from mass spectroscopy suggest an assembly mechanism in solution based on such triplets––the Silver-Trigon Assembly Road (STAR). Indeed, the STAR mechanism may be a general synthetic pathway toward even larger silver polyhedral cages. Besides its fundamental appeal, this synthetic cage may be considered for use as a molecular luminescent thermometer.

Abstract

Buckminsterfullerene (C ₆₀) 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 Ag ₁₈₀ 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 C ₆₀, topologically a truncated icosahedron, an Archimedean solid with icosahedral ( I _h) point-group symmetry. If C ₆₀ can be described mathematically as a curling up of a 6.6.6 Platonic tiling, the Ag ₁₈₀ cage can be described as a curling up of a 3.4.6.4 Archimedean tiling. High-resolution electrospray ionization mass spectrometry reveals that {Ag ₃} _n subunits coexist with the Ag ₁₈₀ species in the assembly system before the final crystallization of Ag ₁₈₀, suggesting that the silver Trigon is the smallest building block in assembly of the final cage. Thus, we assign the underlying growth mechanism of Ag ₁₈₀ to the Silver-Trigon Assembly Road (STAR), an assembly path that might be further employed to fabricate larger, elegant silver cages.