The EPR spectrum of a Cu(II/II/III) cluster: anisotropic exchange in a bent Cu(II)2O2 core

We have analyzed in detail the EPR spectrum of [(N,N,N′,N′-tetramethyl-trans-(1R,2R)-cyclohexanediamine)3 Cu(II)2Cu(III)O2](trifluoromethanesulfonate)3, a compound containing the unusual Cu3(II/II/III) bis-μ-oxo core, as well as a closely related compound. In this system, the Cu(II) ions are ferromagnetically coupled yielding a S=1 ground state (J=+7 cm−1) that splits due to zero-field splitting. From simulation of the data, the zero-field splitting parameter D is 0.751 cm−1, which is far too large to arise from only spin–spin dipolar coupling with a Cu(II)Cu(II) distance of 2.71 Å. The dominant contribution to the D-tensor is found to be anisotropic exchange. The anisotropic exchange interaction is ferromagnetic and is analyzed in terms of ground state-excited state exchange coupling pathways. The important contributions of the nonplanarity of the Cu(II)2O2 subunit and the covalency of the oxide bridging ligands to these pathways are considered.