Piperazinediium bis(pyridine-2,6-dicarboxylato)nickelate(II) tetrahydrate

The reaction of nickel(II) nitrate hexahydrate with the proton-transfer compound piperazinediium pyridine-2,6-dicarboxylate, or (pipzH(2))(pydc) (in which pipz is piperazine and pydcH(2) is pyridine-2,6-dicarboxylic acid), in aqueous solution leads to the formation of the title compound, (pipzH(2))[Ni(pydc)(2)]center dot 4H(2)O or (C4H12N2)[Ni(C7H3NO4)(2)]center dot 4H(2)O. The anion is a six-coordinate complex with a distorted octahedral geometry around Ni-II. The torsion angles show that the two (pydc)(2-) units are almost perpendicular to each other. Considerable pi-pi stacking interactions between two aromatic rings of (pydc)(2-), with distances of 3.4686 (14) and 3.5034 (14) angstrom, are observed. Extensive intermolecular O-H center dot center dot center dot O, N-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonding involving the (pydc)(2-) ligand, (pipzH(2))(2+) as counter-ion and uncoordinated water molecules connect the various components into a supramolecular structure.