Structures and properties of coordination polymers with a rigid zwitterionic pyridinium-based tricarboxylate ligand

Four coordination polymers of different transition metal ions with a new zwitterionic tricarboxylate ligand, 1-(3,5-dicarboxylatophenyl)-4-carboxylatopyridinium (L²⁻) were synthesized, and characterized. They are formulated as [Cu₃L₂(OH)₂] (1), [M(L)(H₂O)] [M = Mn (2) and Cd (3)] and [Zn(L)(H₂O)]·2H₂O (4). Compound 1 exhibits a 3D coordination framework containing carboxylate- and hydroxo-bridged chains. Compounds 2 and 3 are isomorphic and both contain 2D coordination layers based on carboxylate-bridged dinuclear units (Mn or Cd). Compound 4 shows a different 2D coordination layer, and extensive hydrogen bonding and π-π stacking interactions lead to a 3D architecture with 1D channels, in which square tetraaqua clusters are captured through hydrogen bonds. The compound undergoes a structural transformation upon dehydration and the structure can be switched back through reabsorbing water. Magnetic studies demonstrated that the mixed μ-OH and μ-1,3-carboxylate bridges in 1 mediate ferromagnetic exchange owing to orbital countercomplementarity. The carboxylate bridges in 2 mediate weak antiferromagnetic interactions. Compounds 3 and 4 show the fluorescence characteristic of the ligand.