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

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 C13H16- N-2[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 mu-oxalato-kappa O-3(1),O-2:O-1 ' 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 center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds and weaker C-H center dot center dot center dot O interaction to form a 3D net. In contrast to 2, the emerging building blocks are linked into 1D chains via N- H center dot center dot center dot O hydrogen bonds, which further extend to form a 3D supramolecular framework through Cu-O-axial and other C-H center dot center dot center dot 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 degrees 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.