We synthesized and X-ray characterized four complexes based on pyrazine- and pyridine-dicarboxylic acid ligands. One of them exhibited a relevant lp–π-hole interaction.

H9a-Acr) 2 [Ni(hypydc) 2 ]•4H 2 O (1), (H9a-Acr) 2 [Co(hypydc) 2 ]•3H 2 O (2), (H9a-Acr) [Cr(hypydc) 2 ]•3H 2 O (3), and (H9a-Acr) 2 [Cd(hypydc) 2 ]•3H 2 O ( 4) compounds (H 2 hypydc = 2,6-dicarboxy-4-hydroxypyridine or chelidamic acid; 9a-Acr = 9-aminoacridine) were synthesized via proton transfer and characterised by elemental analysis, IR spectroscopy, and single crystal X-ray diffraction techniques. Thermogravimetric analysis (TGA) was carried out on compounds 1 and 2. Compounds 1-4 have distorted octahedral geometries with two hypydc 2− ions coordinated as tridentate ligands to each metal ion through one oxygen atom of each carboxylate group and the nitrogen atom of the pyridine ring. In the compounds, strong hydrogen bonds between anionic, cationic, and water fragments and especially the different natures of the π + ⋯π − and π − ⋯π − stacking interactions play important roles in the construction of threedimensional supramolecular frameworks which have been analysed in detail. The smaller trans N-M-N angle observed in 3 as compared with the others has been analysed using DFT calculations. Solution studies have also been performed to understand the behaviour of the ternary systems M n+ -H 2 hypydc-9a-Acr in aqueous solution.

In the title hydrated mixed-cation salt, (C5H8N3)2[Co(H2O)6](SO4)2·2H2O, the complete octa­hedral hexa­aqua complex cation is generated by crystallographic inversion symmetry. In the crystal, the components are linked by O—H⋯O and N—H⋯O hydrogen bonds, the latter, involving pyrimidinium cations and sulfate anions, generating R 2 2(8) loops. These, together with π–π inter­actions between centrosymmetrically related pyrimidinium cations [centroid–centroid separation = 3.5460 (8) Å], lead to the formation of a three-dimensional network.