Synthesis and magnetism of ferromagnetically coupled [CuII3CrIII] heterotetranuclear complexes with N,N′-bis(aminoalkyl)oxamidatocopper(II)

The strategy of "complex as ligand" allowed us to synthesize two new mu-oxamido-bridged copper(II)-chromium(III) heterotetranuclear complexes formulated as [Cu-3(oxae)(3)Cr](CIO4)(3) (1) and [Cu-3(oxpn)(3)Cr](CIO4)(3) (2), where oxae and oxpn represent N,N'-bis(2-aminoethyl)oxamido and N,N'-bis(3-aminopropyl)oxamido dianions, respectively. Based on elemental analyses, molar conductivity and magnetic moment (room-temperature) measurements, IR, ESR and electronic spectral studies, these complexes are proposed to have oxamido-bridged structures consisting of three copper(II) ions and a chromium(III) ion, in which the chromium(III) ion has an octahedral environment and the three copper(II) ions have a square-planar environment. The two heterotetranuclear complexes were further characterized by variable-temperature magnetic susceptibility (3 similar to 300 K) measurements and the magnetic data have been used to deduce the indicated heterotetranuclear structure. The results derived from least-squares fit of the experimental data have confirmed the ferromagnetic interaction between the adjacent copper(II) and chromium(III) ions through the oxamido-bridge within each molecule. On the basis of the spin Hamiltonian operator, H = -2J(S-Cul (.) S-Cr + S-Cu2 (.) S-Cr + S-Cu3 (.) S-Cr), the magnetic analyses were carried out for the two copper(II)-chromium(III) heterotetranuclear complexes and the spin-coupling constants (J) were evaluated as +13.98 cm(-1) for (1) and +12.65 cm-1 for (2). The results indicate that the bridging oxamido should be able to transmit ferromagnetic interaction in the strict orthogonality [(Cu3CIII)-C-II] system. The influence of the symmetry of the magnetic orbitals on the nature of the magnetic interaction between the paramagnetic centers is preliminarily discussed.