EPR INVESTIGATIONS OF TRINUCLEAR CU(II) COMPLEXES WITH QUARTET GROUND-STATES

The X-band EPR powder spectrum of is[mu-(2-diethylamino)ethanolato-O,N ]bis(methanol) tricopper(II) (CuEtBz . MeOH) at liquid-helium temperat ure is dominated by a resonance with g' approximate to 4.3 (the prime indicates effective g-values). This signal shows a very unusual temper ature dependent resonance shift with a variation of the effective g-va lue to g' approximate to 2.6 at room temperature. Single-crystal EPR i nvestigations 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-fie ld EPR spectra recorded with a resonance frequency nu = 244.99 GHz, no resonance shifts were observed. Based on this observation and on simu lations of the temperature dependence of the X-band signal, it was pos sible 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 exper iment. Therefore the observed signal embodies the contributions of eac h spin state, yielding a temperature-dependent resonance field due to the varying thermal populations of the different levels.