Planar Chirality and Helical Polymers: Ferrocenyl-Substituted Amidinium–Carboxylate Salts

A series of amidinium–carboxylate salts, [FcC{N(Cy)H][O2CR′′] (Fc = ferrocenyl, R′′ = Me, tBu, Ph, mesityl, CF3) have been synthesized and structurally characterized. In each case, the amidinium cation forms two NH···O hydrogen bonds to carboxylate anions. The extent of aggregation is, however, dependent on the carboxylate substituent. For R′′ = Me, a 1:1 salt is observed with an {E,E}-configuration of cyclohexyl groups within the amidinium component, whereas R′′ = tBu forms a 2:2 dimeric salt with an {E,Z}-cation. This latter configuration generates a planar chiral cation. When R′′ = Ph or mesityl, the hydrogen-bonding forms a helical polymer, with each chain containing a single enantiomeric form of the cation. In contrast, when R′′ = CF3 the hydrogen-bonded polymer contains alternating enatiomeric forms of the cation.