Synthèse, caractérisation, comportement magnétique et topologie de complexes métalliques d’oxalate(2-)

Abstract

Within the scope of this work, four new hybrid salts involving [CuC 2O4 )2 ]2- anions and pyridiniums cations, (C 5H7 N2 )2[Cu(C 2 O4 )2 ]∙2H2 O (Cu-1), (C 7 H11N2 )2 [Cu(C2 O4 )2 ]∙5H2 O (Cu-2), (C 5 H7 N2 O)2[Cu(C2 O4 )2] (Cu-3), C 12 H16N2[CuC 2 O4 )2] (Cu-4), were synthesized and characterized by various analytical techniques, especially : melting point, microelemental analysis, scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDX), vibrational and electronic spectroscopies, electron paramagnetic resonance (EPR), thermal analysis (TGA et DSC) and single crystal X-ray diffraction (XRD). Magnetic studies have also been conducted. The Cu-1 compound with dark blue and elongated crystals crystallizes in the Monoclinic system, space group P21/c. The Cu-2 compound appears as prismatic crystals with a sky-blue coloration and crystallizes in the Triclinic system, space group P-1. The dark green crystals of the Cu-3 compound have a shape comparable to that of rice grains. It also crystallizes in the Triclinic system, space group P-1. The crystals of the Cu-4 compound are large blocks with dark blue color, crystallizing in the Monoclinic system, space group P21/c. The coordination environment around the Cu(II) metal center appears differently in each of these coordination compounds. In Cu-1, Cu(II) is bound to four oxygen atoms from two oxalate(2-) ligands located in the equatorial plane and interacts weakly with two axial O atoms of neighboring [Cu(C2 O4 )2]2- units forming a CuO 6 prolate octahedron. In Cu-2, Cu(II) is in a disorded octahedral environment formed by six oxygen atoms from three (C 2 O4 )2- ions with a well-established helical orientation. The Cu(II) in the Cu-3 compound is bound to four oxygen atoms from two oxalate(2-) ligands located in the same plane and at nearly identical distances, therefore the Cu(II) is in this complex in a square-planar environment. In the last compound, Cu-4, Cu(II) is a square base pyramidal geometry. It is symmetrically bis-chelated by four oxygen atoms from two oxalate(2-) ligands located in the same plane and bonded axially to a single oxygen atom from the [Cu(C2 O4 )2 ]2-anionic entity. On the other side, we noted distinct polymerization mode of the [Cu(C2 O4 )2 ]2- unit. In Cu-1, the [Cu(C2 O4 )2]2- unit polymerizes via a double O-bridge, resulting in a linear Cu(II) chain; in Cu-2, the polymerization of this unit takes place via the oxalate(2-) molecule, resulting in a zigzag chain of Cu(II); we don’t assist to a polymerization phenomenon in the Cu-3 hybrid salt. In contrast to its predecessors, the Cu-4 hybrid salt exhibits a polymerization of the [Cu(C 2 O4 )2]2- unit via a single O-bridge leading to a Cu(II) zigzag chain. The energy dispersive spectra indicated the presence of carbon, oxygen, nitrogen and copper in all compounds. The electron spectra of these salts revealed absorption bands characteristic of d-d transitions. Thermal studies indicated the presence of crystallization water molecules in the Cu-1 and Cu-2 salts and their absence in the Cu-3 and Cu-4 salts. They also revealed that Cu-1 and Cu-2 materials are thermally stable up to 280°C; Cu-3 and Cu-4 up to 180°C. The magnetic studies revealed weak antiferromagnetic and ferromagnetic interactions between Cu(II) ions in Cu-1 and Cu-2, respectively. The topological analysis of tris-oxalate, [M′M(ox) 3 ] x- complexes led to five topological networks other than those so far encountered in the literature (2D hcb-net and 3D srs-net). These are the 3D lig-, nod-, ths-nets (already known from the literature as encountered in compounds other than tris-oxalates, [M′M(ox)3]x-) and the 3D noa- and daz-nets (completely new, never obtained as a topological network for any compound).