A New Binding Motif for Amidinium Cations � Strong Binding of Amidinium Salts with m-Xylylene Bisphosphonate Moieties

In contrast to the multitude of different binding motifs for the molecular recognition of ammonium cations very few efficient guanidinium receptors have been reported and even less amidinium hosts [1]. The latter is surprising in view of important role of the amidinium functionality in drugs targeting binding pockets for the arginine sidechain etc. The challenge, however, is quite clear: One has to surround the amidinium group in a halfmoon-like array with at least four hydrogen bond acceptors, which are ideally preoriented for maximum electrostatic as well as hydrogen bond interactions.