n or {(pipzH 2 )[Zn(py-2,3-dc) 2 ]Á-4H 2 O} n , where py-2,3-dcH 2 is pyridine-2,3-dicarboxylic acid and pipz is piperazine, was obtained by the reaction of zinc(II) nitrate tetrahydrate with (pipzH 2 )(py-2,3-dc) as a protontransfer compound in aqueous solution. Each Zn II (site symmetry 1) is coordinated in a distorted octahedral geometry by four O atoms and two N atoms from two bidentate (py-2,3dc) 2À ligands, which also act as bridging ligands between Zn II atoms. The four donor atoms of the two coplanar (py-2,3-dc) 2À anions form a square-planar arrangement around the Zn II centre. In the crystal structure, extensive O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, as well as ion pairing andstacking [with a distance of 3.8693 (8) Å between two aromatic rings] between different fragments, play an important role in the stabilization of the supramolecular structure.

The title compound, (C4H12N2)[V(C2O4)2O(H2O)]·2H2O, has been obtained by the reaction of vanadium(III) chloride with piperazinediium oxalate in aqueous solution. It shows distorted octahedral geometry [V=O = 1.6010 (12) Å and V—O = 1.9932 (12)–2.2176 (12) Å]]. The structure exhibits intermolecular N—H...O and C—H...O hydrogen bonds. The uncoordinated water molecules contribute to the formation of the supramolecular structure via O—H...O hydrogen bonds.

The title compound, [Cu(C7H3NO5)(C12H8N2)(H2O)]·4.5H2O or [Cu(hypydc)(phen)(H2O)]·4.5H2O (phen is 1,10-phenanthroline and hypydcH2 is 4-hydroxy­pyridine-2,6-dicarboxylic acid), was obtained by the reaction of copper(II) nitrate hexa­hydrate with the proton-transfer compound (phenH)2(hypydc) in aqueous solution. Both the cationic and the anionic fragments of the proton-transfer compound are involved in complexation. Each CuII atom has a distorted octa­hedral geometry. It is hexa­coordinated by three O atoms and three N atoms, from one phen fragment (as bidentate ligand), one (hypydc)2− unit (as tridentate ligand) and a water mol­ecule. In the crystal structure, O—H⋯O and C—H⋯O hydrogen bonds, and π–π stacking inter­actions [centroid-to-centroid distance 3.5642 (11) Å] between the phen ring systems, contribute to the formation of a three-dimensional supra­molecular structure.

The title polymeric compound, [Cd(C7H3NO4)(H2O)3]n or [Cd(py-2,3-dc)(H2O)3]n, where py-2,3-dcH2 is pyridine-2,3-dicarboxylic acid, was obtained by the reaction of cadmium(II) nitrate hexa­hydrate with (pipzH2)(py-2,3-dc) as a proton-transfer compound in aqueous solution (pipz is piperazine). The mol­ecular structure shows that only the anionic fragment of the starting proton-transfer compound is present in the complex, while the (pipzH2)2+ dication has been lost. Each (py-2,3-dc)2− ligand bridges two CdII atoms in two different coordination modes, i.e. one end acts as a monodentate and the other end as a bidentate ligand. The three remaining coordination sites on the metal center are occupied by water mol­ecules. The geometric arrangement of the six donor atoms around the CdII atom is distorted octa­hedral. In the crystal structure, O—H⋯O and C—H⋯O hydrogen bonds play an important role in stabilizing the supra­molecular structure.

Key indicators: single-crystal X-ray study; T = 100 K; mean (C-C) = 0.002 Å; disorder in solvent or counterion; R factor = 0.032; wR factor = 0.090; data-toparameter ratio = 18.0.The reaction of magnesium(II) chloride tetrahydrate with the proton-transfer compound piperazinediium pyridine-2carboxylate, (pipzH 2 )(pyc) 2 (pipz is pirerazine and pycH is pyridine-2-carboxylic acid) in aqueous solution leads to the formation of the title compound, [Mg(C 6 H 4 NO 2 ) 2 -(H 2 O) 2 ]Á0.15H 2 O. The Mg atom is six-coordinated in a distorted octahedral environment by two bidentate pyridine-2-carboxylate groups and two O atoms of coordinated water molecules, which are located in cis positions. In the crystal structure, intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, and -[-distance = 3.5616 (8) Å for pyridine rings; symmetry code: 2 À x; 1 À y; 1 À z] and C-HÁ Á Á stacking, connect the various components into a supramolecular structure.