Copper Complexes of the Biginelli-Type Ligand: Effect of Solvents and Halide Ions on Crystal Structure

A series of copper(I) halide complexes coordinated by a cyclic thiourea ligand featuring methyl, acetyl, and m-nitrophenyl substituents has been synthesized and structurally elucidated. These complexes were studied using a combination of X-ray single-crystal and powder diffraction, IR, NMR, and UV-vis spectroscopy, alongside DFT calculations. The results revealed a remarkable structural diversity dictated by the halide anion and the solvent used. Copper(I) chloride and bromide form mononuclear complexes with a CuHalL₂ composition, while copper(I) iodide forms polynuclear complexes in a 1:1 stoichiometry, exhibiting a unique polymorphism. CuI forms complexes with both polymeric chains and cyclic structures, depending on the solvent used for synthesis and crystallization. DFT calculations were used to characterize the molecular structure and absorption spectra of the complexes. Application of simplified TD-DFT approximation allowed us to assign UV-vis absorbance bands to metal-to-ligand charge transfer (MLCT) transitions. These findings demonstrate a powerful approach to controlling the nuclearity and architecture of copper(I) coordination complexes through variations in anions and crystallization conditions.