A layered copper/bis(4-pyridylformyl)piperazine coordination polymer with cyclic water molecule octamers and reversible structural reorganization

► Layered copper coordination polymer with bis(4-pyridylformyl)piperazine) (4-bpfp) ligands. ► Unique hydrogen-bonded R8 cyclooctane-like octameric water molecule clusters. ► Crystal-to-crystal transformation with color change on dehydration and rehydration.