Alkali metal complexes of aromatic polycarboxylates -: a balance of π-stacking and coordinate bonding interactions?

Crystal structure determinations of, in most cases, hydrated, alkali metal derivatives of the dicarboxylic acids, 2,2'-bipyridine-3,3'-dicarboxylic acid (H2BDC) and chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic acid, H2CHEL) show numerous similarities, such as in the predominance of O-coordination in generating solid state polymers in which parallel arrays of the essentially planar ligand ring units are apparent, though not necessarily indicative of conventional pi-stacking interactions, and some unanticipated differences. In particular, all species derived from chelidamic acid, including its diammonium compound, appear to be complexes of the partially deprotonated pyridone form of this ligand. In both systems, close contacts between atoms constituting the aromatic entities take a variety of forms depending upon the associated metal.