Molecular modeling and crystal structure analysis of the mixed-anion sodium copper selenite chloride Na₂Cu₈(SeO₃)₈Cl₂

A new mixed-anion sodium copper selenite chloride, Na₂Cu₈(SeO₃)₈Cl₂, was successfully synthesized via a hydrothermal method and structurally elucidated using single-crystal X-ray diffraction. The compound crystallizes in the tetragonal space group P4/ncc and forms a three-dimensional framework built from trigonal pyramidal SeO₃^2- units with stereochemically active lone electron pairs, Jahn-Teller-distorted CuO₄Cl coordination polyhedra exhibiting a distorted vacant octahedral (vOC-5) geometry, and irregularly coordinated Na⁺ cations. The interplay between the stereoactivity of Se(IV) lone pairs and the coordination flexibility of Cu²⁺ ions is crucial for the stabilization and architecture of the crystal structure. Intermolecular interactions play a significant role in the crystal packing, with the largest Hirshfeld surface contributions arising from Cu···O, Na···O, and O···O contacts, while enrichment ratio analysis identifies Se···Cl and Na···O contacts as the most strongly preferred relative to a random distribution of surface atoms. Spectroscopic investigations (FT-IR and UV-Vis) confirm the presence of selenite groups and characteristic Cu²⁺ electronic transitions. Spectroscopic investigations (FT-IR and UV-vis) confirm the presence of selenite groups and characteristic Cu²⁺ electronic transitions, while the weak O-H-related infrared bands are attributed to adsorbed atmospheric moisture rather than structurally incorporated water. Thermal analysis reveals a multistep decomposition process involving removal of adsorbed moisture, limited oxygen release, progressive decomposition of the selenite groups with SeO₂ evolution, and the formation of sodium-containing and copper oxide residue phases. The exact composition of the final residue cannot be established from TG-DSC measurements alone. These findings contribute to expanding the class of mixed-anion selenite-halide materials and highlight the relationship between structure, bonding, and thermal stability.

The crystal structure of Na₂Cu₈(SeO₃)₈Cl₂ was determined by single-crystal X-ray diffraction analysis. Intermolecular interactions and crystal packing features were investigated using Hirshfeld surface analysis and enrichment ratio calculations, allowing quantification of key contact contributions. Reduced density gradient (RDG) analysis was performed in Multiwfn using the promolecular electron density generated directly from the experimental crystal structure (Independent Atom Model), allowing visualization of strong coordination interactions and weaker dispersive contacts without additional quantum-chemical calculations. Infrared spectroscopy was used to identify vibrational modes associated with SeO₃^2- groups, while UV-vis spectroscopy provided insight into the electronic transitions of Cu²⁺ centers. Thermal behavior was examined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), enabling the identification of distinct decomposition stages and correlation with theoretical mass-loss calculations. The combined crystallographic, spectroscopic, Hirshfeld surface, RDG, and thermal analyses provide a comprehensive understanding of the structural, intermolecular, spectroscopic, and thermal properties of the compound, supporting its relevance in the study of Se(IV)-based functional inorganic materials.