Supramolecular co-ordination networks constructed via pi-stacking interactions and charge-assisted hydrogen bonds

The diprotic acid 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, H2SQA) reacts with [(eta5-C5H5)2Co] in THF to form a hydrogen bonded co-ordination network in crystalline {[CoII(HSQA-HSQA)2(H2O)4]2-}{[(eta5-C5H5)2CoIII]+}2 (1) which contains CoII and CoIII metal centres and the supramolecular salt {[(eta5-C5H5)2CoIII]+}{[(HSQA)(H2SQA)]-} (2) depending on the solvent and on the stoichiometric ratio. The diprotic acid pyridine-2,6-dicarboxylic acid (dipicolinic acid, H2DPA) reacts with [(eta5-C5H5)2Co] and [(eta6-C6H6)2Cr] in THF or water to form the supersalt {[(eta5-C5H5)2Co]+}{ [(HDPA)(H2DPA)]-} (3) and the co-ordination complex {[(eta6-C6H6)2CrI]+}2{[(DPA)2CrIII]-}.2H2O (4), respectively. All compounds have been structurally characterised by single-crystal X-ray diffraction. The metal centres in the mixed valence species 4 are the paramagnetic 17 electrons CrI and 15 electrons CrIII, respectively. The crystalline edifices in all compounds are held together by pi-stacking interactions involving the flat organic moieties and by hydrogen bonding interactions of the O-H...O and/or O-H...O(-) type between the acid moieties, whether as metal co-ordinated ligands or as free ions and by charge assisted C-Hdelta+...Odelta- bonds between cations and anions.