Cage Keeps C₆₀ Captive In The Solid State

Molecular cages can be the rudest of hosts. Guests are welcome as long as there’s plenty of solvent around, but removing the cages from solution and close packing keeps visitors out. Now, chemists in Japan have created a molecular cage capable of capturing fullerene guests, even in the solid state (Nat. Chem., DOI: 10.1038/nchem.742). The University of Tokyo’s Makoto Fujita, Yasuhide Inokuma, and Tatsuhiko Arai created a “crystalline molecular sponge” from a network of octahedral M₆L₄ cages, where the metal complex M is Co(NCS)₂ and the ligand L is 2,4,6-tris(4-pyridyl)-1,3,5-triazine. The material has three distinct cage structures, possessing vast interstitial voids with the cuboctahedral cage structures M₁₂L₈ and M₁₂L₂₄, which are capable of capturing C₆₀ and larger fullerenes. When crystals of the molecular cage were soaked in a saturated toluene solution of C₆₀, they absorbed an impressive 35% of the material’s weight in the fullerene, gradually turning black in the process. Furthermore, when the crystals were placed in a solution that was equal parts C₆₀ and C₇₀, the material preferentially absorbed the larger fullerene, which, according to the authors, could lead to new methods for enrichment and isolation.