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

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-C2 O4 )][Cr(phen)(C2 O4 )2 ]2 ·4H2 O [Mn2 Cr2 (1), Co2 Cr2 (2), Ni2 Cr2 (3), Cu2 Cr2 (4) and Zn2 Cr2 (5)]. Their structural analysis reveals that all the prepared compounds crystallize in the triclinic system, space group P1[combining macron], 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 Cu2 Cr2 (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 Cu2 Cr2 (4), whereas in Mn2 Cr2 (1), Co2 Cr2 (2) and Ni2 Cr2 (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.