Pronounced effects of zero-point energy difference on intramolecular electron transfer in asymmetric mixed-valence biferrocenium cations:: Structural, EPR, and 57Fe Mossbauer characteristics

The X-ray structural determinations of a series of asymmetric mixed-valence biferrocenium cations have been determined. The observations of the structural characteristics of the series of mixed-valence biferrocenium cations are also consistent with our Mossbauer studies. The features in the Mossbauer spectra of the asymmetric biferrocenium salts include two doublets, one with a quadrupole splitting of 1.5-2.2 mms(-1) (Fe(II) site) and the other with a quadrupole splitting of 0.7-0.5 mms(-1) (Fe(III) site). This pattern of two doublets is expected for a mixed-valence biferrocenium cation, which is valence-trapped on the time scale of the Mossbauer technique. Our data indicate that the zero-point energy difference plays a crucial role in determining the nature of the intramolecular electron-transfer rate. The electrochemical measurements and the EPR data are also described.