ELECTRON-TRANSFER IN MIXED-VALENCE BIFERROCENIUM SALTS - EFFECT OF ZERO-POINT ENERGY DIFFERENCE AND PRONOUNCED ANION DEPENDENCE

The effects of zero-point energy and counterion on the rate of intramolecular electron transfer in mixed-valence biferrocenium salts are investigated. All asymmetrically substituted biferrocenium triiodide salts, where the substituent is either 1'-ethyl (9), 1'-butyl (10), 1'-acetyl (11), 1'-butyryl (12), or 1'-ethyl 6'-propyl (13), are found to be localized on the Mossbauer, EPR, and IR time scales. In 300 K Mossbauer spectra they each show two doublets, one for Fe(II) metallocene and the other for Fe(III) metallocene (electron-transfer rates less than approximately 10(7) s-1). The cation in each of compounds 9-13 is not symmetric; that is, the two irons are not in equivalent environments. This asymmetry induces a nonzero zero-point energy barrier for intramolecular electron transfer. The effects on the rate of electron transfer of replacing I3- by picrate (14), IBr2- (15), and PF6- (16) in 2 are also examined. Replacing I3- by picrate and IBr2- leads to a decrease in the rate of electron transfer. The Mossbauer spectrum taken at 300 K for 14 consists of two doublets. In other words, the electron-transfer rate of 14 is less than approximately 10(7) s-1. In the case of 15, the variable-temperature (100-275 K) Mossbauer spectra exhibit a localized electronic structure. The PF6- salt is converted from valence trapped at low temperatures to valence detrapped above 280 K. The difference in electron-transfer rates can be explained by the magnitude of the energy barrier of charge oscillation in anions and cation-anion interactions.