Synthesis and characterization of two isostructural 3d-4f coordination compounds based on pyridine-2,6-dicarboxylic acid and 4,4'-bipyridine.

The design and synthesis of 3d-4f heterometallic coordination polymers have attracted much interest due to the intriguing diversity of their architectures and topologies. Pyridine-2,6-dicarboxylic acid (Hpydc) has a versatile coordination mode and has been used to construct multinuclear and heterometallic compounds. Two isostructural centrosymmetric 3d-4f coordination compounds constructed from pyridine-2,6-dicarboxylic acid and 4,4'-bipyridine (bpy), namely 4,4'-bipyridine-1,1'-diium diaquabis(μ-pyridine-2,6-dicarboxylato)tetrakis(pyridine-2,6-dicarboxylato)bis[4-(pyridin-4-yl)pyridinium]cobalt(II)dieuropium(III) octahydrate, (CHN)[CoEu(CHN)(CHNO)(HO)]·8HO, (I), and 4,4'-bipyridine-1,1'-diium diaquabis(μ-pyridine-2,6-dicarboxylato)tetrakis(pyridine-2,6-dicarboxylato)bis[4-(pyridin-4-yl)pyridinium]cobalt(II)diterbium(III) octahydrate, (CHN)[CoTb(CHN)(CHNO)(HO)]·8HO, (II), were synthesized under hydrothermal conditions and characterized by IR and fluorescence spectroscopy, thermogravimetric analysis and powder X-ray diffraction. Both compounds crystallize in the triclinic space group P-1. The Eu and Tb cations adopt nine-coordinated distorted tricapped trigonal-prismatic geometries bridged by three pydc ligands. The Co cation has a six-coordination environment formed by two pydc ligands, two bpy ligands and two coordinated water molecules. Adjacent molecules are connected by π-π stacking interactions to form a one-dimensional chain, which is further extended into a three-dimensional supramolecular network by multipoint hydrogen bonds.