Synthesis, crystal structure, thermal analysis and magnetic properties of two bis(oxalato)cuprate(II) hybrid salts crystallizing with lattice oxalic acid molecules

Abstract

Two novel bis(oxalato)cuprate(II) salts, (C7H11N2)2[Cu(C2O4)2]·H2C2O4 (1) and (C5H7N2)2[Cu(C2O4)2(H2O)2]·2H2C2O4 (2) {(C7H11N2)+ = 4-dimethylaminopyridinium, (C5H7N2)+ = 3-aminopyridinium} have been synthesized and characterized by elemental and thermal analyses, single-crystal X-ray diffraction, IR spectroscopy and SQUID magnetometry. Compounds 1 and 2 crystallize in the triclinic P-1 space group. The two structures differ in terms of the coordination ligands around the Cu(II) centers as well as the number of the lattice oxalic acid molecules. The crystal structure of 1 is made of columnar stacks of [Cu(C2O4)2]2- anions through axial Cu∙∙∙O contacts (2.89 Å), yielding straight Cu(II) chains with a prolate CuO6 octahedron around Cu(II) ions. In this chain structure, the intermetallic Cu∙∙∙Cu separation between two adjacent Cu(II) ions is 3.71 Å. The crystal structure of 2 is characterized by the [Cu(C2O4)2(H2O)2]2- complex anion in which aqua ligands are in trans-configuration, with the Cu(II) ion adopting a distorted (4 + 2) octahedral coordination. In the two compounds, the ionic components together with the lattice oxalic acid molecules are connected via intermolecular NH∙∙∙O and OH∙∙∙O hydrogen bonds into a three-dimensional network. In addition, π–π stacking interactions between pyridine rings contribute to the stabilization of the crystal packing. Temperature-dependence susceptibility measurements investigated in the temperature range 2–300 K revealed weak antiferromagnetic coupling at low temperatures in both compounds.