Crystal engineering with coordination compounds of 2,6-dicarboxy-4-hydroxypyridine and 9-aminoacridine fragments driven by different nature of the face-to-face π...π stacking

H9a-Acr)(2)[Ni(hypydc)(2)]center dot 4H(2)O (1), (H9a-Acr)(2)[Co(hypydc)(2)]center dot 3H(2)O (2), (H9a-Acr) [Cr(hypydc)(2)]center dot 3H(2)O (3), and (H9a-Acr)(2)[Cd(hypydc)(2)]center dot 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 pi(+)... pi(-) and pi(-)...pi(-) stacking interactions play important roles in the construction of three-dimensional 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 Mn+-H(2)hypydc-9a-Acr in aqueous solution.