Synthesis and magnetic studies of μ-oxalato-bridged copper(II)-M(III)-copper(II) (M=Cr and Fe) heterotrinuclear complexes

Two new μ-oxalato-bridged CuII2MIII-type (M=Cr and Fe) heterotrinuclear complexes have been prepared and identified as [Cu2Cr(ox)3(tmen)2]ClO4 (1) and [Cu2Fe(ox)3(tmen)2]ClO4 (2), where ox represents the oxalato dianions; tmen stands for N,N,N′,N′-tetramethylethylenediamine. The two complexes have not yet been isolated in crystalline form suitable for X-ray structure analysis, but based on elemental analyses, molar conductivity and magnetic moment (at room-temperature) measurements, IR, ESR and electronic spectra studies, it is proposed that these complexes have oxalato-bridged structure consisting of two copper(II) ions and a chromium(III) or an iron(III) ion, in which the chromium(III) or iron(III) ion has an octahedral environment, and the two copper(II) ions have a square-planar environment. The two heterotrinuclear complexes were further characterized by variable temperature magnetic susceptibility (4.2–300 K) measurements and the magnetic data have been also used to deduce the indicated heterotrinuclear structure. The results derived from least-squares fit of the experimental data have confirmed that the adjacent copper(II) and chromium(III) ions through the oxalato-bridge in complex (1) are ferromagnetically coupled with the exchange integral J(Cu–Cr)=+10.2 cm−1, while an intramolecular antiferromagnetic coupling is detected between the copper(II) and iron(III) ions through the oxalato-bridge in complex (2) with the exchange integral J(Cu–Fe)=−11.6 cm−1, on the basis of the spin Hamiltonian operator, H =−2J( S Cu1 · S M + S Cu2 · S M ) (M=Cr3+ or Fe3+).