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

Abstract

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 57 Fe Mössbauer spectroscopy (time scale ∼10 7 s - 1 ). The features in all of the 80 K spectra include two doublets, one with a quadrupole splitting (Δ E Q ) of ∼2 mm s - 1 (Fe(II) site) and the other with Δ E Q = ∼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 Mössbauer experiment (electron-transfer rate < ∼10 7 s - 1 in the solid state). Increasing the sample temperature in 1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocenium triiodide, 1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocenium triiodide, and 1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocenium triiodide causes the two doublets to move together with no discernible line broadening, eventually becoming a single “average-valence” doublet at temperatures of 295, 265, and 190 K, respectively. In the case of 1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocenium triiodide, the two doublets at low temperatures do not coalesce into an average-valence doublet at 300 K (electron-transfer rate < ∼10 s - 1 at 300 K). Thus, the micromodification of the relative positions of the alkyl substituents and the nature of the alkyl substituents has a dramatic influence on the rate of intramolecular electron transfer. The deviations of the Cp rings from the parallel position correlate quite well with the Mössbauer critical temperature for electronic delocalization−localization in mixed-valence cations. Qualitatively, we suggest that the metal nonbonding orbitals (d x 2 - y 2, d xy ) start to interact with the ligand π orbitals as the Cp rings tilt from the parallel position. On the basis of the density functional calculations, a theoretical explanation of the influence of bending back the Cp rings on the electron-transfer rates is presented. The electrochemical measurements, IR data, and near-IR spectra for these mixed-valence biferrocenium cations are also presented.