The reaction of Mn(II) salts with N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine yields a dinuclear Mn2L3 complex, which on the basis of its structural and magnetic characterisation can be formulated as a Mn(IV) derivative of both dinegative bis-iminosemiquinonato and mixed valence trinegative iminosemiquinonato-iminocatecholato forms of the reacting ligand. HF-EPR spectra at various frequencies suggest that the electron transfer process in this mixed valence species is frozen on the time scale of EPR at high frequency, whereas is still dynamic at lower frequencies. This spectral behaviour therefore indicates a thermally activated character of the electron transfer process, as expected in a class 11 mixed valence system. (C) 2002 Elsevier Science B.V. All rights reserved.