Syntheses, Structures and Electrochemical Properties of Two New Copper(II) Complexes Based on Isomeric Bis(pyridylformyl)piperazine Ligands and Rigid/Flexible Organic Dicarboxylates

Two new copper(II) complexes, [Cu₂(3-bpfp)(2,6-PDA)₂(H₂O)₂] (1) and [Cu(4-bpfp)_0.5(glu)]·H₂O (2), have been hydrothermally synthesized by self-assembly of isomeric bis(pyridylformyl)piperazine ligands [3-bpfp = bis(3-pyridylformyl)piperazine, 4-bpfp = bis(4-pyridylformyl)piperazine], rigid pyridine-2,6-dicarboxylic acid (2,6-H₂PDA) or flexible glutaric acid (H₂glu), and copper(II) chloride. Single-crystal X-ray diffraction analysis reveals that two adjacent Cu^II ions are connected by the 3-bpfp ligand to build a dinuclear unit in complex 1, in which 2,6-PDA serves as a terminal chelating ligand. Adjacent dinuclear units are further linked by hydrogen bonding and π-π stacking interactions to form a three-dimensional (3D) supramolecular network. Complex 2 is a 3D coordination polymeric framework based on a layer polymer [Cu(glu)]_n and bridging 4-bpfp ligands with 6-connected (4⁴.6¹⁰.8) topology. In 1 and 2, the ligands 3-bpfp and 4-bpfp adopt a μ₂-bridging coordination mode (via ligation of pyridyl nitrogen atoms). The thermal stability and the electrochemical properties of the title complexes have been studied.