190448-32-9Relevant academic research and scientific papers
Pronounced effects of ring tilting on rates of intramolecular electron transfer in polyalkyl-substituted mixed-valence biferrocenium triiodides
Dong, Teng-Yuan,Lee, Shu-Hwei,Chang, Chung-Kay,Lin, Hsiu-Mei,Lin, Kuan-Jiuh
, p. 2773 - 2786 (2008/10/08)
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, l′,2′,1?,2?-tetrabenzylbiferrocene, l',3',l',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 M?ssbauer 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 M?ssbauer experiment (electron-transfer rate 7 s-1 in the solid state). Increasing the sample temperature in l′,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?-tetrapropyl-biferrocenium triiodide, the two doublets at low temperatures do not coalesce into an average-valence doublet at 300 K (electron-transfer rate -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 M?ssbauer critical temperature for electronic delocalization-localization in mixed-valence cations. Qualitatively, we suggest that the metal nonbonding orbitals (dx2-y2, dxy) 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.
