A binuclear Cu(II) metallacycle capable of discerning between pyrazine and its different methyl-substituted derivatives based on reversible intracage metal-ligand binding.

Squashy cage: A flexible, spongelike, and reversible metal-binding Cu(2)L(2) cage that can adjust its internal space in response to pyrazine and its derivatives based on a recognition sequence of pyrazine, 2,5-dimethylpyrazine, and 2-methylpyrazine (see figure) is described.