The influence of metal centres on the exchange interaction in heterometallic complexes with oxalate-bridged cations

Abstract

The reaction of bis(phenanthroline)metal(II) cations (M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) with bis(oxalato)chromium(III) anions in a water/ethanol solution gives rise to a series of compounds with oxalate-bridged cations, [{M(phen)₂}₂(μ-C₂O₄)][Cr(phen)(C₂O₄)₂]₂·4H₂O [Mn₂Cr₂ (1), Co₂Cr₂ (2), Ni₂Cr₂ (3), Cu₂Cr₂ (4) and Zn₂Cr₂ (5)]. Their structural analysis reveals that all the prepared compounds crystallize in the triclinic system, space group P, having similar unit cell parameters, molecular structures and crystal packing features. All metal centres in 1–5 are octahedrally coordinated: M²⁺ in homodinuclear cations are coordinated with two phen molecules and one bridging oxalate ligand; Cr³⁺ in anions is coordinated with one phen ligand and two bidentate oxalate groups. The copper atom in Cu₂Cr₂ (4) exhibits a Jahn–Teller-distorted octahedral coordination. Owing to the considerable number of pyridyl groups present in 1–5 (from phen ligands) the crystal packing of cations and anions is driven by stacking interactions appearing in offset-face-to-face (OFF) and edge-to-face (EF) orientations. The hydrogen bonds between the anions and water molecules of crystallization form 1D ladder-like motifs. In addition to the single crystal X-ray diffraction studies, the characterization of the new complexes was accomplished by means of IR and UV/Vis spectroscopy and magnetization measurements on a SQUID magnetometer. The temperature dependence of magnetic susceptibility reveals different magnetic super exchange interactions taking place in homodinuclear oxalate-bridged cations depending on the transition metal centre (Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺). Oxalate ligands mediate the ferromagnetic coupling of Cu²⁺ metal cations in Cu₂Cr₂ (4), whereas in Mn₂Cr₂ (1), Co₂Cr₂ (2) and Ni₂Cr₂ (3), antiferromagnetic interactions are observed between Mn²⁺, Co²⁺ and Ni²⁺ cations, respectively. Also, relatively large zero-field splitting parameters for Cr³⁺ ions (from mononuclear anions), D ≈ 1 cm⁻¹, were observed.