Crystal structures and properties of Fe(H₂O)₂[Ni(CN)₄]nH₂O hydrates

We report the crystal structures and properties of iron hydrate compounds. These hydrates have 2D topologies and crystallize with three different structural features depending on the number of weakly bonded water molecules. Coordinated water molecules occupy the axial sites of the iron ion and support a network of hydrogen-bonded water molecules in the interlayer region. Crystal structures of these hydrates are known for Mn, Co, and Ni but not for Fe. The structure for the solids considered herein was solved and refined from powder XRD patterns complemented by IR, UV/Vis-NIR, Mossbauer, and TG data. The relevance of this structural study is related to the application of Hofmann-like coordination polymers as spin-crossover materials when the coordinated water molecules are replaced by dipyridines, pyridine, and derivatives that form pillars between adjacent layers. Upon dehydration of these solids, a reactive material is formed by assembly of tetrahedral iron ions with square planar nickel ions.