The mixed ligand complexes {[Cu(4Cl-Phe)(bpy)(H₂O)]Cl·[Cu(4Cl-Phe)(bpy)Cl]·2H₂O} (1) and [Cu(4HPG)(bpy)]·2H₂O (2) were synthesized and their crystal structures determined by X-ray crystallography (4Cl-Phe = D,L-4-chlorophenylalaninato, 4HPG = D-4-hydroxyphenylglycinato, bpy = 2,2′-bipyridine). The asymmetric unit of crystals of 1 consists of two slightly different Cu(II) complexes-one contains an axial water ligand, whereas the second one has a chloride anion at that position. By contrast, 2 exhibits two close similar molecules, both having axial chloride ligands, and non coordinated water. The geometrical parameters for π,π-stacking interactions were determined by an application of PLATON program. Intermolecular π,π-interaction between bpy ligands on adjacent Cu(II) moieties results in homo pairs of complexes in the case of 1. In addition, both species of compound 1 exhibit intramolecular interligand π,π-interaction between the 4Cl-phenyl ring of 4Cl-Phe and the metalloaromatic Cu–bpy chelate ring. In crystal 2, multi-stacked homo chains are formed. Analogous calculations for previously reported, related compounds, similar to those presented here, revealed that in most cases with intra-molecular interligand π,π-interaction there is an efficient “aryl–metal chelate ring” π,π-stacking instead of an aryl–aryl interligand π,π-interaction, which indeed is not observed. These findings let us to take a critical look at the nature of the intramolecular interligand π,π-stacking interaction in the referred copper(II) mixed-ligand complexes.

This work deals with the preparation of pyridine-3-carbohydrazide (isoniazid, inh) cocrystals with two α-hydroxycarboxylic acids. The interaction of glycolic acid (H₂ga) or d,l-mandelic acid (H₂ma) resulted in the formation of cocrystals or salts of composition (inh)·(H₂ga) (1) and [Hinh]⁺[Hma]^–·(H₂ma) (2) when reacted with isoniazid. An N′-(propan-2-ylidene)isonicotinic hydrazide hemihydrate, (pinh)·1/2(H₂O) (3), was also prepared by condensation of isoniazid with acetone in the presence of glycolic acid. These prepared compounds were well characterized by elemental analysis, and spectroscopic methods, and their three-dimensional molecular structure was determined by single crystal X-ray crystallography. Hydrogen bonds involving the carboxylic acid occur consistently with the pyridine ring N atom of the isoniazid and its derivatives. The remaining hydrogen-bonding sites on the isoniazid backbone vary based on the steric influences of the derivative group. These are contrasted in each of the molecular systems. Finally, Hirshfeld surface analysis and Density-functional theory (DFT) calculations (including NCIplot and QTAIM analyses) have been performed to further characterize and rationalize the non-covalent interactions.