EPR spectrum features of the tetrameric copper compounds of formula Cu4OX6L4

The recent experimental observation of S=2 EPR signals from tetrameric Cu(II) complexes having the general formula Cu4OX6L4 is interpreted on the basis of a static quasi-Heisenberg model for the complex, where the anisotropic symmetric and skew-symmetric interactions have been taken into account with the approximation of the second order of the perturbation theory. It is found that the observed g value fits the single-ion mean value g=(gCu∥+2gCu⊥)/3, as expected, and the constant for the cubic zero-field splitting may be simply written as B4=(D2a+D2b+D 2c)/120J0, where J0 is the isotropic exchange coupling constant and Da,b,c are the principal values of the symmetric component of the anisotropic interaction. It is found, in contrast, that both the symmetric and skew-symmetric parts may contribute to the not yet observed S=1 signals. When the model is applied to the case of Cu4OCl6(TPPO)4 the B4 value measured by EPR and the J0 value derived by magnetic susceptibility data seem to support a symmetric exchange component very large compared with the dipolar energy.