Counterion effects on the intramolecular electron-transfer rate of mixed-valence biferrocenium salts: micromodulation and phase transitions

The effect of changing the counterion from I,to 12Br-, Br21-, or PF6upon the rate of intramolecular electron transfer in the mixed-valence biferrocenium cation is examined. Room temperature powder X-ray diffraction data are presented to show that the 1,Brand BrJsalts are isostructural to biferrocenium triicdide. The 4.2 K X-band EPR spectrum of biferrocenium BrzIis axial (g,, = 3.60 and g, = 1.75) and very similar to that for the Issalt. However, the g-tensor anisotropy (g,, = 3.23 and g, = 1.90) observed for the 4.2 K EPR spectrum of biferrocenium 12Bris appreciably reduced compared to that of the I,and BrJ salts. This probably reflects the low-symmetry environment provided by the asymmetric I-I-Branion. The 57Fe Mossbauer characteristics of biferrocenium IzBrresemble those of the 1,salt; however, everything is shifted to lower temperatures. The 150 K spectrum essentially consists only of two doublets (one Fe" and one Fell'). A third average-valence doublet grows in with increasing temperature Until at -340 K only this average doublet is seen. A temperature of only -220 K is needed to give only an average-valence doublet for the Br21salt. Thus, replacing the 1,by the Br21anion leads to a dramatic reduction by 150 deg in the temperature where the mixed-valence biferrocenium cation transfers an electron faster than the Mossbauer technique can sense. A qualitative model is developed to explain the effect of the anion replacement. The importance of the cation-cation and cation-anion interactions, as well as the intrinsic charge-oscillation barrier heights in the mixed-valence cations and anions, is discussed relative to the phase transition that is believed to be present in these compounds. In very recent studieszw7 it has been shown that the solid-state environment plays a crucial role in determining the rate of intramolecular electron transfer in various mixed-valence biferrocenium salts. Triiodide salts (Y= 13-) were studied. The