Oxalate-Bridged Dinuclear Cr(III)-M(II) (M = Cu, Ni, Co, Fe, Mn) Complexes: Synthesis, Structure, and Magnetism

The following oxalate-bridged heterodinuclear Cr(III)-M(II) (M = Cu, Ni, Co, Fe, Mn) complexes have been synthesized: [Cr(salen)(ox)Cu(acpy)] (1) (salen = N,N′-ethylenebis(salicylideneaminate), ox²⁻ = oxalate ion, acpy = N-acetylacetonylidene-N-(2-pyridylethyl)aminate) and [Cr(salen)(ox)M(taea)](BPh₄) (M = Ni (2), Co (3), Fe (4), Mn (5)) (taea = tris(2-aminoethyl)amine). The [Cr(salen)(ox)Cu(acpy)]2DMFN·MeOH (1′) complex crystallizes in the monoclinic system of the space group P2₁/n with a = 16.429(6) Å, b = 25.303(14) Å, c = 10.191(4) Å, β = 109.78(2)°, V = 3986(3) ų, and Z = 4. The refinement converges with R = 0.079 and R_w = 0.071 based on 3327 reflections with |F_o| ≥ 3σ(|F_o|). The complex has an oxalate-bridged dinuclear Cr(III)-Cu(II) core with a Cr⋯Cu distance of 5.482(3) Å. The Cr(III) has a cis-β octahedral geometry with the tetradentate salen in the folded form and a bidentate oxalate group. The geometry around the Cu(II) ion is square-pyramidal with the three donor atoms of acpy and one of the oxalate oxygens at the basal plane and the other oxygen of the oxalate group at the apex. Magnetic investigations of 1–5 in the 4.2–300 K temperature range reveal a ferromagnetic interaction between the Cr(III) and M(II) ions for all the complexes. On the basis of the spin Hamiltonian, Ĥ = −JS_Cr·S_M, the values for the spin coupling constant J were estimated to be +2.8, +4.6, +1.3, +0.8, and +0.5 cm⁻¹ for 1–5, respectively. The relative magnitude of the J values is explained by the σ- and π-pathways through the oxalate bridge. The correlation between the J values of 1–5 and the phase-transition temperatures, T_C, of the ferromagnetic {NBu4[MCr(ox)₃]}_x (M = Cu, Ni, Co, Fe, Mn) is discussed based on Heisenberg's ferromagnet model.