Nature of copper(II)-lanthanide(III) magnetic interactions and generation of a large magnetic moment with magnetic anisotropy of 3d-4f cyclic cylindrical tetranuclear complexes

A cyclic cylindrical 3d-4f tetranuclear structure, in which the 3d and 4f magnetic ions are arrayed alternately, has been found to be a suitable molecular design to produce a large magnetic moment and large magnetic anisotropy. Complexes 3-10 with the chemical formula [MLLn(hfac)₂]₂ ((M^II, Ln^III) = (Cu, Eu) (3), (Cu, Gd) (4), (Cu, Tb) (5), (Cu, Dy) (6), (Ni, Eu) (7), (Ni, Gd) (8), (Ni, Tb) (9), (Ni, Dy) (10)) have been synthesized, where H₃L = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxybenzylideneamino)ethane and Hhfac = hexafluoroacetylacetone. The powder X-ray diffractions and FAB-mass spectra demonstrated that these complexes assume a similar tetranuclear structure. The crystal structures of 4 and 5 showed that each complex has a cyclic cylindrical tetranuclear Cu^II₂Ln^III₂ structure, in which the Cu^II complex functions as a "bridging ligand-complex" to two adjacent Ln^III ions. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations at 2 K from 0 to 5 T have been measured for four pairs of Cu^II₂Ln^III₂ and Ni^II₂Ln^III₂, in which compound Ni^II₂Ln^III₂ containing diamagnetic Ni^II ion was used as the reference complex to evaluate the Cu^II-Ln^III magnetic interaction. Comparison of the magnetic properties of the Cu^II₂Ln^III₂ complex with those of the corresponding Ni^II₂Ln^III₂ complex showed that the magnetic interaction between Cu^II and Eu^III ions is weakly ferromagnetic and that between Cu^II and either of Gd^III, Tb^III, and Dy^III ions is ferromagnetic. Complex Cu^IIGd^III₂, 4, has an S = 8 spin ground state, due to the ferromagnetic spin coupling between S_Gd = 7/2 and S_Cu = 1/2 with coupling constants of J₁ = +3.1 cm^-1 and J₂ = +1.2 cm^-1. The magnetic measurements showed that compounds 5 and 6, Cu^II₂Ln^III₂ (Ln^III = Tb, Dy), exhibit large magnetic moments and large magnetic anisotropy due to the Ln^III ion.