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

[Sm-2(fumarate)(3)(H2O)(4)]center dot 3H(2)O, a new porous pillared layer framework with OD 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.