On the importance of π-stacking interactions in the complexes of copper and zinc bearing pyridine-2,6-dicarboxylic acid N-oxide and N-donor auxiliary ligands

Abstract

Three coordination complexes {[Cu(pydco)(pz)_0.5(H₂O)]·H₂O}_n (1), [Cu(pydco)(phen)(H₂O)]·2H₂O (2), and [Zn₂(pydco)(phen)₂Cl₂]·2H₂O (3) have been synthesized at room temperature using a mixed-ligand system including pyridine-2,6-dicarboxylic acid N-oxide (H₂pydco) as O-donor as well as pyrazine (pz) and 1,10-phenanthroline (phen) as N,N′-bidentate donors. Single-crystal X-ray diffraction (XRD) revealed that 1 forms a 1D-coordination polymer through bridging pz units while 2 and 3 form discrete complexes in which endo-bidentate phen can only connect with one metal centre via a five-membered chelate ring. Also, pydco²⁻ can form a six-membered chelate ring around the metal ion via the N-oxide oxygen and one carboxylate oxygen. However, another carboxylate group of this ligand can also participate in the coordination and create unique structures in 1 and 3. Subsequently, 1–3 can expand their structures into 2D-supramolecular networks through π⋯π stacking or H-bonding interactions. Recurrent π⋯π stacking motifs in all compounds have been further analyzed energetically using DFT calculations and characterized using the quantum theory of “atoms-in-molecules” (QTAIM) and noncovalent interaction plot (NCIPlot) index computational tools. In compound 3, cooperativity effects in π⋯π/π⋯π/π⋯π assemblies have been also analyzed.