The new hexadentate N₂O₄ Schiff base ligand from condensation of 2,4-hydroxybenzophenone and 1,3-propanediamine has been prepared in methanol and ethanol solutions. The Schiff base synthesized from EtOH contains half molecule of amine per one molecule of ligand in its structure. The complexes were synthesized from the direct reaction of this ligand and Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) acetates. The Schiff base and its complexes were characterized by elemental analysis, X-ray crystallographic techniques, spectroscopic (UV–Vis, IR) and thermal (TG, TG-FTIR) methods, molar conductance and magnetic measurements. The complexes, except for Ni(II) compound, similar to the Schiff base are amorphous solids. The Ni(II) complex crystallizes in the monoclinic space group P2₁/c. The N₂O₂ coordination geometry around nickel(II) is slightly tetrahedrally distorted square planar. Variable temperature magnetic studies of the paramagnetic complexes show the existence of weak antiferromagnetic [for complexes of manganese(III) and cobalt(II)] and weak ferromagnetic [for copper(II)] interactions. The complexes are stable at ambient temperature. After heating at first they lose solvent molecules (water or methanol), and after that organic part undergoes defragmentation and combustion. Based on the obtained data from the TG-FTIR analysis, the main gaseous products resulting from thermal degradation in nitrogen atmosphere are CO₂, CO, NH₃, acetic acid, methane, C₆H₆, C₆H₅CN and C₆H₅CH₃.

In recent years, the crystal engineering library has been enriched with a number of previously unrecognized or unnoticed intermolecular interactions, such as agostic, tetrel, chalcogen, pnicogen bonding and chelate ring stacking – collectively referred to as “unconventional interactions”. Many open questions remain unaddressed regarding their ability to form synthon interactions, specificity, and cooperativity, for example with π–π stacking interactions. In this work, we throw light on the formation of chelate ring stacking in metal–organic assemblies of nicotinohydrazide ligands (N′-(1-(2-pyridyl)ethylidene)nicotinohydrazide (HL) and N′-(phenyl(pyridin-2-yl)methylene)nicotinohydrazide (HL₁)) with mercury(II) halide (HgBr₂, HgI₂) salts. Their reaction produced five compounds, namely [Hg(μ-L)BrHgBr₂]_n (1), [Hg(μ-L₁)Br]_n (2), [Hg(L)I₂] (3), [Hg(HL₁)I₂]·(CH₃OH) (4), and [Hg(μ-L₁)I]_n (5). Crystal structure analysis reveals that chelate ring stackings are formed in four of the reported metal–organic compounds, and are common also in the literature precedents. The energies of chelate ring stackings and π–π heterocycle stackings have been computed and analyzed by means of DFT calculations, and the results were verified using Bader's theory of “atoms in molecules”. These results provide a rationale for preferential formation of both unconventional and conventional stackings and allow us to conclude that chelate ring interaction may be considered as a synthon interaction for nicotinohydrazide metal complexes. Interpretations for packing differences imposed by the substituent effect (substitution of methyl group in HL for phenyl group in HL₁) were provided based on the Hirshfeld surface analysis and 2D fingerprint plots of the crystal structures reported here.

The new mono-, di- and tetranuclear coordination compounds [Cu(HL¹)]·H₂O (1), [Cu₂(L¹)(OAc)(MeOH)]·2H₂O·MeOH (2), [Cu₄(L²)₂(OAc)₂]·4MeOH (3), and [Cu₄(L²)₂(OAc)₂]·4H₂O·4MeOH (4) were synthesized by the direct reaction of 2,2′-{(2-hydroxypropane-1,3-diyl)bis[nitrilomethylidene]}bis(4-bromo-6-methoxyphenol) (H₃L¹) or 2,2′-{(2-hydroxypropane-1,3-diyl)bis(nitriloeth-1-yl-1-ylidene)}diphenol (H₃L²) and the Cu(II) salt. They were characterized by elemental analysis, X-ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy, simultaneous thermal analysis and differential scanning calorimetry (TG/DSC), and thermal analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) techniques and the single crystal X-ray diffraction study. In the dinuclear complex 2, the copper(II) ions are bridged by an alkoxo- and a carboxylato bridges. The tetranuclear complexes 3 and 4 are formed from dinuclear species linkage through the phenoxo oxygen atoms of the fully deprotonated H₃L². Compounds 1–4 are stable at room temperature. During heating in air, at first, the solvent molecules (water and/or methanol) are lost and after that, the organic part undergoes defragmentation and combustion. The final decomposition solid product is CuO. The main gaseous products resulting from the thermal degradation of 1–4 in a nitrogen atmosphere were: H₂O, MeOH, CH₃COOH, CH₄, C₆H₅OH, CO₂, CO, and NH₃.

N,N′-bis((2-hydroxyphenyl)(phenyl)methylidene)propane-1,2-diaminato-N,N′,O,O′)-(nitrato-O)-manganese(III) methanol solvate ([Mn(C₂₉H₂₄N₂O₂)(NO₃)CH₃OH]) was synthesized and characterized by FTIR, UV–Vis, TG–FTIR, TG/DSC, molar conductivity, magnetic moment measurement and single crystal X-ray analysis. In the structure, the Mn(III) ion adopts a distorted octahedral geometry with two nitrogen and two oxygen atoms from the Schiff base ligand in the equatorial plane, and the nitrate ion and methanol molecule in the axial position. The effects of organic solvents of various polarities on the UV–Vis spectra of the ligand and complex were investigated. The manganese(III) complex is easily dissolved in organic polar aprotic solvents and has moderate solubility in organic polar protic solvents.

The complexes of N₂O₂-, N₂O₃- and N₂O₅-donors Schiff bases with Cu^II ions in the methanol solution have been synthesized. They were characterized by elemental analysis, X-ray crystallographic techniques, spectroscopic (UV–Vis, IR) and thermal (TG, TG-FTIR) methods. The catalytic activity of the prepared complexes for the hydrogen peroxide-assisted degradation of methylene blue (MB) as the model compound in water was also investigated. The complexes were obtained as crystalline solids. Depending on the Schiff bases and the molar ratio of Cu^II:ligand used in the synthesis, they can form monomeric or dimeric structures. The coordination environment around the metal centre in both solution and solid states is a slightly distorted square-planar. The values of magnetic moments determined at room temperature show the existence of antiferromagnetic interactions. The complexes are stable at ambient temperature. After heating, at first solvates lose solvent molecules; after that, an organic part undergoes gradual defragmentation and combustion. Degradation efficiency of MB in the presence of complexes was found to be 68.18–97.47%. A tentative mechanism involving HO· radical as an oxidant for degradation of MB was proposed.