Synthesis, structure and magnetic properties of two bis(oxalato)cuprate(ii) salts with pyridinium type counter ions

Abstract

Mono- and a di-protonated pyridine type cations have been used for the synthesis of two bis(oxalato)cuprate(II) salts, namely, (C₅H₇N₂O)₂[Cu(C₂O₄)₂] (C₅H₇N₂O = 2-amino-3-hydroxypyridinium) 1 and C₁₃H₁₆N₂[Cu(C₂O₄)₂] (C₁₃H₁₆N₂ = 4,4′-trimethylenedipyridinium) 2. In the crystal structures, C₂O₄²⁻ adopts different coordination modes: the common bidentate chelating oxalate in 1 and the relatively scarce μ-oxalato-κ³O¹,O²:O¹′ in 2. X-ray diffraction also revealed that [Cu(C₂O₄)₂]²⁻ anions do not polymerize in 1 (a phenomenon hardly observed in such salts), while in 2, the polymerization of [Cu(C₂O₄)₂]²⁻ units occurs via the Cu–O_axial contact forming a zigzag Cu(II) chain. In complex 1, the emerging building blocks are linked into 2D supramolecular layers via N–H⋯O and O–H⋯O hydrogen bonds and weaker C–H⋯O interaction to form a 3D net. In contrast to 2, the emerging building blocks are linked into 1D chains via N–H⋯O hydrogen bonds, which further extend to form a 3D supramolecular framework through Cu–O_axial and other C–H⋯O interactions. The structural diversities show that iminium counterions play key roles in the construction of various architectures. Thermal analyses showed no weight loss for either system in the temperature range of 20–180 °C, which indicates that none of these complexes possess solvation water molecules. Magnetic studies indicate the presence of antiferromagnetic coupling between the spin centres in 1 and 2.