THE EFFECT OF INTERANNULAR BRIDGE ON ELECTRON-TRANSFER RATE IN MIXED-VALENCE BIFERROCENIUM TRIIODIDE SALTS

The physical properties of the new mixed-valence 1',3:1''',3"-bis(propane-1,3-diyl)-1,1"-biferrocenium triiodide (1) and 1',3:1''',3"-bis(pentane-1,5-diyl)-1,1"-biferrocenium triiodide (2) are reported. Compound 1 gives a Mossbauer spectrum with a single "average-valence" doublet at 77 K. On the other hand, compound 2 gives a Mossbauer spectrum with two quadrupole-split doublets at 300 K. Furthermore, compound 1 shows relatively EPR isotropic g tensors compared to compound 2. Therefore, compound 1 is judged to have an electron-transfer rate in excess of approximately 10(10) s-1, whereas the electron-transfer rate for 2 is slower than approximately 10(7) s-1. Both compounds are localized on the IR timescale. Thus, the change of interannular bridge in biferrocenium cations leads to a dramatic influence on the rate of electron transfer. It is concluded that compound 1 has a higher degree of electronic coupling between two iron ions than compound 2. This difference in rate possibly originates in a structural difference in the interannular bridge in 1 and 2.