Tunable supramolecular synthons and versatile, water-soluble building blocks for crystal engineering: [(eta(5)-C5H4COOH)(2)Co-III](+) and its zwitterionic form [(eta(5)-C5H4COOH) (eta(5)-C5H4COO)Co-III]

It is shown that the water-soluble dicarboxylic cationic acid [(eta (C5H4CO OH)-C-5)2Co(III)](+) (1) is an extremely versatile building block for the c onstruction of organometallic crystalline edifices. Removal of one proton f rom 1 leads to formation of the neutral zwitterion [(eta (5)-C5H4COOH)(eta (5)-5H4COO)Co-III] (2), while further deprotonation leads to formation of t he dicarboxylate monoanion [(eta (5)-C5H4COOH)(2)Co-III]- (3). Compounds 1, 2 and 3 possess different hydrogen-bonding capacity and participate in a v ariety of hydrogen-bonding networks. The cationic form 1 has been character ised as its [PF6](-) and Cl- salts 1-[PF6] and 1-Cl .H2O, as well as in its co-crystal with urea, 1-Cl.3(NH2)(2)CO, and with the zwitterionic form 2, [(eta (5)-C5H4COOH)(eta (5)-C5H4COO)Co-III] [(eta (5)-C5H4COOH)(2)Co-III]()[PF6](-), 2.1-[PF6]. The neutral zwitterion 2 behaves as a supramolecular crown ether: it encapsulates the alkali cations K+, Rb+ and Cs+ as well as the ammonium cation NH4+ in cages sustained by O-H . . .O and C-H . . .O hy drogen bonds to form co-crystalline salts of the type 2(2)-M[PF6] (M = K, R b, Cs) and 2(2)-[NH4][PF6]. The deprotonated acid 3 has been characterised as its Cs+ salt, Cs+-3 .3 H3O.