Phthalic acid, a versatile building block in organic-organometallic crystal engineering

Phthalic acid [C6H4-1,2-(COOH)(2), H(2)PA] and terephthalic acid [C6H4-1,4- (COOH)(2), H(2)TPA] have been reacted with aqueous solutions of the hydroxi des [(eta(5)-C5H5)(2)Co](+)[OH](-) and [(eta(6)-C6H6)(2)Cr](+)[OH](-) produ ced in situ by oxidation of the parent neutral molecules. The acid-base rea ction leads to self-assembly of the deprotonated acid anions into honeycomb superstructures held together by hydrogen-bonding interactions of the O-H ... O and charged O-H ... O- types. The superanions accommodate the [(eta(5 )-C5H5)(2)Co](+) and the paramagnetic [(eta(6)-C6H6)(2)Cr](+) organometalli c cations via charge-assisted C--Hdelta+... Odelta- hydrogen bonds. Four no vel organic-organometallic cocrystals, namely {[(eta(5)-C5H5)(2)Co](+)}(4){ [HPA](-)}(2)[PA](2-) . 4H(2)O (1), [(eta(6)-C6H6)(2)Cr](+)[HPA](-)[H(2)PA] (2), {[(eta(5)-C5H5)(2)Co](+)}[TPA](2-). 6H(2)O (3) and {[(eta(6)-C6H6)(2)C r](+)}(2)[TPA](2-). 6H(2)O (4) have been isolated and structurally characte rized by low-temperature X-ray diffraction measurements. It is shown that p hthalic acid is a very versatile building block in the formation of hydroge n-bonded networks and unprecedented superanionic architectures. The role pl ayed by water molecules in the stabilization of the crystal structures in t he absence of all or almost all acidic protons is discussed.