A Crystallographic Story of Snakes and Ladders: Encapsulated Water Polymers and Rigid Metal Complexes Based on Mono-N-substituted Carboxylate 4,4'-Bipyridine

A new zwitterionic bidentate ligand was synthesized incorporating a 4,4'-bipyridine core with a carboxylate group as a single building block, specifically designed for the rational construction of supramol. structures (metal-org. frameworks) that are capable of encapsulating and aligning water aggregates leading to the formation of extended polymeric networks. In particular, both the terminal N' end and a carboxylate oxygen of the monosubstituted 4,4'-bipyridine was found to readily coordinate in a straightforward manner with different transition metal halides in water, affording heterocyclic complexes featuring various structural motifs. However, crit. to the stability and shape of the incorporated water structures is the presence of strong hydrogen bonding resulting from the directional influence exerted by an unbound oxygen center of the metal-bonded carboxylate groups. A search of the Cambridge Structural Database reveals that only a small no. of complexes that feature a free carboxylate oxygen strongly interact with a hydrogen atom assocd. with a cis-bonded aqua ligand.