Tunable supramolecular synthons and versatile, water-soluble building blocks for crystal engineering:

It is shown that the water-soluble dicarboxylic cationic acid (eta5-C5H4COOH)2Co(III)]+ (1) is an extremely versatile building block for the construction of organometallic crystalline edifices. Removal of one proton from 1 leads to formation of the neutral zwitterion (eta5-C5H4COOH)(eta5-C5H4COO)Co(III)] (2), while further deprotonation leads to formation of the dicarboxylate monoanion (eta5-C5H4COO)2Co(III)]- (3). Compounds 1. 2 and 3 possess different hydrogen-bonding capacity and participate in a variety of hydrogen-bonding networks. The cationic form 1 has been characterised 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)2CO, and with the zwitterionic form 2, (eta5-CH4COOH)(eta5-C5H4COO)Co(III)](eta5-C5H4COOH)2Co(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 hydrogen bonds to form co-crystalline salts of the type 2(2)-MPF6] (M = K, Rb, Cs) and 2(2)-NH4]PF6]. The deprotonated acid 3 has been characterised as its Cs+ salt, Cs+-3.3H2O.