AminidiniumÀcarboxylate bridges are important in protoncoupled electron transfer in biological systems 1 and have been widely used in the construction of supramolecular assemblies. 2 In the latter context, they have been identified in double helices, 3,4 a triple stranded "molecular braid", 5 cylindrical complexes, 6 molecular capsules, 7 and an optically active [2]catenane. 8,9 Among the reasons for the widespread occurrence of this linkage are the high association constant for charge-assisted hydrogen bonds (CAHBs, A, Figure 1), and the supposed "well-defined geometry" linking the two components together. The first point has merit, 10 but because trisubstituted amidinium cations can adopt three possible configurations (BÀD, Figure 1), 11,12 and hydrogen bonds to carboxylates can be syn, anti, and nonplanar, 13 the geometry of the bridge may be considerably more varied.Incorporation of redox-active centers in hydrogen-bonded ion pairs is important in many applications, with most work on amidiniumÀcarboxylate pairs in the area of electrochemically controllable chemosensors. 14 It has been shown that ferrocenecarboxylic acid forms strong hydrogen-bonded bridges with a benzamidine, 15 and a series of related [3.3]ferrocenophanes, in which ferrocene is N-bound to a guanidine, have been used in the selective recognition of anions, cations, and amino acids. 16 In 1997, the synthesis of N,N 0 -dicyclohexyl-C-ferrocenyl amidine was reported (I-H, Scheme 1) 17 and a series of coordination compounds incorporating the corresponding amidinate anion, [I] À , have been described. 17,18 The structure of the protonated guanidinium, [I-H 2 ] þ , has not been reported. Herein we describe the synthesis and structure of a series of carboxylate salts and show that, depending on the carboxylate substituent, [I-H 2 ] þ may be planar chiral in the solid-state and involved in the formation of helical polymeric chains.
