Bis(guanidinium) bis(4-hydroxypyridine-2,6-dicarboxylato-k(3)O(2),N,O-6)nickelate(II) dihydrate

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