A 3D porous lanthanide-fumarate framework with water hexamer occupied cavities, exhibiting a reversible dehydration and rehydration procedure.

[Sm2(fumarate)3(H2O)4] x 3 H2O, a new porous pillared layer framework with 0D cavities for the accommodation of chair-like hexameric water clusters, possesses three kinds of fumarate ligand with their two COO ends adopting different coordination modes and shows reversible de- and re-hydration behavior.