Building large, hollow, dodecahedral cages from inorganic building blocks is challenging. This is due to their large cavity, which lessens structural stability. It also is difficult to find building blocks that provide the angles and connectivities required to build the edges of the twelve pentagons that make up a dodecahedron. Polyoxometalates (POMs) are commonly used building blocks in inorganic chemistry, but large polyhedral cages with open sides and high symmetry made from POMs are still rare.

Shiping Yang, Shanghai Normal University, China, Jiang Liu, Nanjing Normal University, China, Ya-Qian Lan, Nanjing Normal University and South China Normal University, Guangzhou, China, and colleagues have synthesized a hollow Mo₂₄₀-based dodecahedron with open sides (pictured). The cage was synthesized via a hydrothermal reaction between (NH₄)₆Mo₇O₂₄, CoSO_4, and NH₂NH₂·2HCl in the presence of trace amounts of dilute H₂SO₄. The product has a formula of (NH₄)₂₁H⁺_59–2x[Mo^V₁₈₀Mo^VI₆₀(OH)₆₀O_620–x(SO₃)_20–x(SO₄)_x], with approximately 337 additional H₂O molecules per cage.

The team used X-ray crystallography to investigate the structure of the cage and found that it forms a dodecahedral structure with T_h symmetry. It has a large inner cavity with a diameter up to 1.8 nm. This cage is composed of 20 [Mo₆O₂₂(SO₃)]ⁿ⁻/[Mo₆O₂₁(SO₄)]^_n− building blocks, which form three connections each at the corners of the cage, and 30  [Mo₄O₁₆]ⁿ⁻ building blocks, which form the edges. According to the researchers, the compound is the highest-nuclearity hollow opening dodecahedral cage with a well-defined crystal structure known so far.

• Self-Assembly of Giant Mo₂₄₀ Hollow Opening Dodecahedra,
  Jiaomin Lin, Ning Li, Shiping Yang, Mingjie Jia, Jiang Liu, Xiao-Min Li, Lu An, Qiwei Tian, Long-Zhang Dong, Ya-Qian Lan,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c06582