The present study aims at the integration of the “oxalic conversion” route into “green chemistry” for the synthesis of copper oxide nanoparticles (CuO-NPs) with controllable structural, morphological, and magnetic properties. Two oxalate-containing precursors (H₂C₂O₄.2H₂O and (NH₄)₂C₂O₄.H₂O) and different volume ratios of a mixed water/glycerol solvent were tested. First, the copper oxalates were synthesized and then subjected to thermal decomposition in air at 400 °C to produce the CuO powders. The purity of the samples was confirmed by X-ray powder diffraction (XRPD), and the crystallite sizes were calculated using the Scherrer method. The transmission electron microscopy (TEM) images revealed oval-shaped CuO-NPs, and the scanning electron microscopy (SEM) showed that morphological features of copper oxalate precursors and their corresponding oxides were affected by the glycerol (V/V) ratio as well as the type of C₂O₄²⁻ starting material. The magnetic properties of CuO-NPs were determined by measuring the temperature-dependent magnetization and the hysteresis curves at 5 and 300 K. The obtained results indicate the simultaneous coexistence of dominant antiferromagnetic and weak ferromagnetic behavior.

In the structure of the title compound, (C₅H₇N₂)[Cr(C₂O₄)₂(H₂O)₂], two crystallographically independent formula units are present. Both chromium atoms are six-coordinated in a distorted octa-hedral geometry by two chelating equatorial oxalato ligands and two axial water mol-ecules. The [Cr(C₂O₄)₂(H₂O)₂]⁻ anions and C₅H₇N₂ ⁺ cations are linked through a complex three-dimensional hydrogen-bonding network consisting of N-H⋯O and O-H⋯O inter-actions.

In the crystal structure of the title compound, (C₅H₆ClN₂)[Cr(C₂O₄)₂(H₂O)₂]·1.5H₂O, the Cr^III atom adopts a distorted octahedral geometry being coordinated by two O atoms of two cis water molecules and four O atoms from two chelating oxalate dianions. The cis-diaquadioxalatochromate(III) anions, 2-amino-5-chloropyridinium cations and uncoordinated water molecules are linked into a three-dimensional supramolecular array by O—H...O and N—H...O hydrogen-bonding interactions. One of the two independent lattice water molecules is situated on a twofold rotation axis.

The title organic–inorganic hybrid salt, (C₇H₁₁N₂)₂[V(C₂O₄)₂O(H₂O)]·2H₂O, shows a distorted octahedral coordination environment for the vanadium(IV) atom in the anion (point group symmetry 2), with four O atoms from two symmetry-related chelating oxalate dianions and two O atoms intransconfiguration from a coordinating water molecule and a terminal vanadyl O atom. In the crystal, (001) layers of cations and anions alternate along [001]. The anionic layers are built up by intermolecular O—H...O hydrogen bonds involving the coordinating and solvent water molecules. The cationic layers are linked to the anionic layersviaN—H...O hydrogen bonds between the pyridinium group and the non-coordinating O atoms of the oxalate group. The 4-(dimethylamino)pyridinium cations are also engaged in π–π stacking with their antiparallel neighbours [centroid-to-centroid distance = 3.686 (2) Å]. Considering all supramolecular features, a three-dimensional network structure is accomplished.