Structure, magnetism and spin coupling mechanism of cyano-bridged Ln ^III-Fe^III binuclear metal complexes

A series of binuclear metal complexes [Ln(L)₄(H ₂O)₃Fe(CN)₆]·nH₂O (Ce(1), Pr(2), Nd(3), Sm(4), Eu(5), Gd(6), Tb(7), Dy(8), Er(9), L=2-pyrrolidinone) were prepared and the X-ray crystal structures of complexes 4, 6, and 8 were determined. All the compounds consist of an Ln-CN-Fe unit, in which an octahedral coordinated Fe^III is bridged to an Ln^III ion located in square antiprism environment by a cyano group. The magnetic properties of 3, 4, 5, 6, 7, 9 show an overall antiferromagnetic behavior. The fitting to the experimental magnetic susceptibilities of complex 6 gave g = 1.99, J = 0.735 cm^-1, zJ′ = -0.080 cm^-1 on the basis of a binuclear spin system (S_Gd = 7/2, S_Fe = 1/2), revealing a ferromagnetic intra-molecular Gd-Fe interaction and an antiferromagnetic inter-molecular interaction. Results on the quantum chemical density functional theory (DFT) calculation for 6 showed that the calculated magnetic coupling constant is 8.1 cm^-1, supporting the occurrence of weak ferromagnetic intra-molecular interaction in 6. The spin density maps for 6 in the high spin ground state and broken symmetry state were obtained, and the spin-spin coupling mechanism is discussed.