EPR investigations of trinuclear Cu(II) complexes with quartet ground states

The X-band EPR powder spectrum of bis(μ-benzoato-O,O′)-bis(benzoato-O)-bis[μ-(2-diethylamino) ethanolato-O,N]-bis(methanol) tricopper(II) (CuEtBz · MeOH) at liquid-helium temperature is dominated by a resonance with g′ ≈ 4.3 (the prime indicates effective g-values). This signal shows a very unusual temperature dependent resonance shift with a variation of the effective g-value to g′ ≈ 2.6 at room temperature. Single-crystal EPR investigations on this compound at 4.2 K were carried out to determine the effective g-values of the quartet ground state: g′_z = 2.1(1), g′_x = 4.0(1) and g′_y = 4.7(1). In the temperature-dependent high-field EPR spectra recorded with a resonance frequency v = 244.99 GHz, no resonance shifts were observed. Based on this observation and on simulations of the temperature dependence of the X-band signal, it was possible to show that the signal shift occurs because the signals of the three spin states of the molecule are not resolved in the X-band experiment. Therefore the observed signal embodies the contributions of each spin state, yielding a temperature-dependent resonance field due to the varying thermal populations of the different levels.