Bis(guanidinium) bis(4-hydroxypyridine-2,6-dicarboxylato-κ3O2,N,O6)nickelate(II) dihydrate

The reaction of nickel(II) nitrate hexahydrate, guanidine (G) and 4-hydroxypyridine-2,6-dicarboxylic acid (hypydcH2) in a 1:2:2 molar ratio in aqueous solution resulted in the formation of the title compound, (CH6N3)2[Ni(C7H3NO5)2]·2H2O or (GH)2[Ni(hypydc)2]·2H2O. The six donor atoms of the two 4-hydroxypyridine-2,6-dicarboxylate or (hypydc)2− ligands form a distorted octahedral arrangement around the NiII centre. Considerable C—O...π stacking interactions between the CO groups of carboxylate fragments and the pyridine rings of (hypydc)2− with a distance of 3.3212 (8) Å are observed. In the crystal structure, a wide range of noncovalent interactions consisting of hydrogen bonding (of the types O—H...O and N—H...O), ion pairing, and π–π [centroid–centroid distance 3.8037 (5) Å], N—H...π and C—O...π stacking interactions connect the various components into a supramolecular structure