Synthesis and structural characterization of nickel(II) coordination complexes with mixed-ligand systems: exploring π−π stacking and hydrogen bonding in supramolecular assemblies

Two coordination complexes [Ni(Hpydco)₂(bpy)] (1) and [Ni(pydco)(phen)(H₂O)₂]·4.5H₂O 0.5CH₃OH (2) have been synthesized using a mixed-ligand system including pyridine-2,5-dicarboxylic acid N-oxide (H₂pydco) as an O-donor ligand and 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) as a chelating N-donor ligand and NiCl₂.6H₂O as a source for Ni ions. The crystal structures of 1 and 2 form discrete complexes in which extensive π−π stacking interactions between aromatic rings of the N-donor ligands resulted in the formation of one dimensional (1D) chain-like structures. The chains are connected by hydrogen bonds to expand the structures into 2D-supramolecular networks. Hirshfeld surface analysis and Density Functional Theory (DFT) calculations were included to rationalize the relative strength of the π-stacking assemblies observed in both complexes.