Multifrequency high-field EPR study of binuclear Mn(III)Mn(IV) complexes

The results from a multifrequency high-field EPR study of five di-mu-oxo br idged mixed-valence binuclear Mn(III)Mn(IV) complexes are reported. Spectra were obtained at 9, 95, and 285 GHz. The g anisotropy was unambiguously ob servable at 285 GHz. Hyperfine and g tensor values were estimated using spe ctral simulation procedures that cyclically and simultaneously fit the mult ifrequency data. In all five cases, the g tensors of the mixed-valence comp lexes were found to be rhombic. The g tensors were analyzed using the vecto r projection model. Most, but not all, of the g anisotropy originates from the Mn(III) center. The rhombic g tensors result from the low symmetry of t he manganese centers. The size of the effective g anisotropy for a given co mplex was found to be a linear function of the average bond distance betwee n the manganese and axial nitrogens. This relationship can be understood in terms of the influence of tetragonal distortion on the electronic levels o f the Mn(III) center. The frequency-dependent line broadening observed in t hese mixed-valence complexes is explained in terms of the relationship betw een g anisotropy and structure.