Mechanosynthesis of Copper(II) Complexes Involving Diverse Nuclearities, Polymorphs, and Magnetic and Optical Properties

The mechanochemical approach, which involves solvent-free grinding or milling of reactants, has gained attention for its sustainability and efficiency. In this work, a variety of copper(II) complexes (mono-, di-, and polynuclear) [CuBr₂(C₁₃H₁₁N₃)₂] (<b>2a</b>), [Cu₂Cl(C₁₃H₁₁N₃)₈]³⁺·3Cl^-·4CH₃CN·4H₂O (<b>3</b>), [Cu₂Br(C₁₃H₁₁N₃)₈]³⁺·3Br^-·4CH₃CN·8H₂O (<b>4</b>), [Cu₄Cl₆O(C₁₃H₁₁N₃)₄] (<b>5</b>), and [Cu₄Br₆O(C₁₃H₁₁N₃)₄]·4CH₃CN (<b>6</b>) were synthesized by controlling the mole ratio of the ligand <b>L</b> (<b>L</b> = 1-(2-pyridinylmethyl)-1<i>H</i>-benzimidazole) and CuX₂ (X = Cl, Br) in 2:1, 1:2, and 1:1 ratios, respectively, through liquid-assisted grinding. Another polymorph of [CuBr₂(C₁₃H₁₁N₃)₂] (<b>2b</b>) was prepared by mechanically grinding the compound [ZnBr₂(C₁₃H₁₁N₃)₂] (<b>1</b>) with CuBr₂ in a 1:1 ratio. This mechanochemical method enabled the formation of diverse coordination structures and coordination modes. Additionally, polymorphism was observed and successfully controlled in mononuclear copper(II) complexes, highlighting the influence of synthetic conditions on the final crystal structure. Density Functional Theory (DFT) calculations specific to crystalline solid phases provided insights into the relative stabilities of the polymorphs. The synthesized complexes were recrystallized in appropriate solvents to obtain single crystals suitable for single-crystal X-ray diffraction (SC-XRD). These crystals, along with the bulk products, were further characterized by other analytical techniques, including powder X-ray diffraction (PXRD), thermal analysis (DSC and TGA), and magnetic and optical measurements to elucidate their structures and properties.