Pronounced Effects of Ring Tilting on Rates of Intramolecular Electron Transfer in Polyalkyl-Substituted Mixed-Valence Biferrocenium Triiodides

Relatively minor perturbations caused by Cp ring substituents in a series of mixed-valence biferrocenium cations have pronounced effects on the electronic structure and rate of intramolecular electron transfer. The X-ray structures of 1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocene, 1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocene, 1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocene, 1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocene, and their corresponding mixed-valence biferrocenium triiodide salts have been determined at 298 K. The rates of intramolecular electron transfer in the mixed-valence cations were estimated by variable-temperature 57Fe Mossbauer spectroscopy (time scale ∼107 s-1). The features in all of the 80 K spectra include two doublets, one with a quadrupole splitting (ΔEQ) of ∼2 mm s-1 (Fe(II) site) and the other with ΔEQ = ∼0.6 mm s-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 experiment (electron-transfer rate < ∼1...