Mixed valence interactions in di-μ-oxo bridged manganese complexes. Electron paramagnetic resonance and magnetic susceptibility studies

EPR examination of the class II (deeply trapped) mixed valence complexes [(bpy)₂MnO₂Mn(bpy)₂]³⁺ (the bipyndyl(III,IV) dimer) and its phenanthroline analogue in acetonitrile solution verifies that these complexes possess inequivalent Mn ions at room temperature. Isotropic hyperfine structure for two Mn ions is resolved with A₁ = 167 ± 3 G and A₂ = 79 ± 3 G for both complexes. The hyperfine pattern with |A₁| ≅ 2|A₂| and the small g anisotropy are consistent with high-spin Mn(III) antiferromagnetically coupled to Mn(IV), producing an S = 1/2 ground state. At room temperature a rate of less than 10⁸ s^-1 is estimated for the thermally activated intramolecular electron transfer, consistent with an upper limit of 10⁶ s^-1 calculated from Hush's theory. The magnetic susceptibility of the (III,IV) complexes is characteristic of a strongly antiferromagnetically coupled S = (2, 3/2) pair. The temperature dependence of the data was in good agreement with the isotropic Heisenberg exchange Hamiltonian H = -2JS₁S₂, yielding J = -150 ± 7 cm^-1 for the bipyridyl(III,IV) dimer and J = -134 ± 5 cm^-1 for the phenanthroline analogue.