A novel hexagonal honeycomb K–Cr-oxalate anionic network with layers separated by a five-coordinated Cu(II)–pypn complex (pypn = N,N′-bis(2-pyridylmethyl)-1,3-propanediamine). Synthesis, characterisation, spectroscopy and crystal structure of {[KCr(C2O4)3

A novel hexagonal-based honeycomb compound with overall formula {[KCr(C2O4)3][Cu(pypn)(H2O)](H2O)4} is reported in which pypn is with the tetradentate ligand (N,N′-bis(2-pyridylmethyl)-1,3-propanediamine). The [KCr(C2O4)3]2− moiety forms a hexagonal honeycomb structure, while the five-coordinated [Cu(pypn)(H2O)]2+ moiety is located in between the layers, partly filling the holes in the cavities. The synthesis, X-ray crystal structure and some spectroscopic properties are presented. The coordination of Cr(III) is octahedral, with a CrO6 chromophore, and the K+ ion is in a KO6 environment (K–O distances vary from 2.36 to 2.48 Ǻ). The [KCr(C2O4)3]2− layers have the K+ ions in a Λ conformation, while the Cr(III) ions in the Δ conformation. The geometry around the Cu(II) is five-coordinated with four nitrogens from the chelating pypn ligand in a plane and the apical position being occupied by the oxygen atom of the coordinating water molecule. The packing of the cationic and the anionic layers appears to be of special interest.