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] (2a), [Cu2Cl(C13H11N3)8]3+·3Cl-·4CH3CN·4H2O (3), [Cu2Br(C13H11N3)8]3+·3Br-·4CH3CN·8H2O (4), [Cu4Cl6O(C13H11N3)4] (5), and [Cu4Br6O(C13H11N3)4]·4CH3CN (6) were synthesized by controlling the mole ratio of the ligand L (L = 1-(2-pyridinylmethyl)-1H-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] (2b) was prepared by mechanically grinding the compound [ZnBr2(C13H11N3)2] (1) 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.