Piperazinium hexaaquacobalt(II) bis[bis(pyridine-2,6-dicarboxylato)-cobaltate(II)] octahydrate

The title compound, (C4H12N2)[Co(H2O)(6)][Co(C7H3O4)(2)](2)center dot 8H(2)O, was obtained by the reaction of cobalt(II) nitrate hexahydrate with the proton-transfer compound piperazinediium pyridine-2,6-dicarboxylate or (pipzH(2))(pydc) (where pipz is piperazine and pydcH(2) is pyridine-2,6-dicarboxylic acid) in aqueous solution. The anionic complex, [Co(pydc)(2)](2-), features six-coordinate Co-II with a distorted octahedral geometry. The structure also contains hexaaquacobalt(II) cations (site symmetry (1) over bar), and piperazinium (site symmetry (1) over bar) as counter-ions and eight uncoordinated water molecules. The torsion angles indicate that the (pydc)(2-) units are almost perpendicular to each other. In the crystal structure, extensive 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 bonds, as well as ion pairing and distances for pi-pi interactions between anion fragments, play an important role in stabilizing the structure.