The Effect of The Solid-State Environment on Intramolecular Electron-Transfer Rates in Mixed-Valence Complexes (Ferrocene)
Dong, Teng-Yuan
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https://hdl.handle.net/2142/70323
Description
Title
The Effect of The Solid-State Environment on Intramolecular Electron-Transfer Rates in Mixed-Valence Complexes (Ferrocene)
Author(s)
Dong, Teng-Yuan
Issue Date
1986
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Abstract
A detailed and interesting picture of what controls the electron transfer between the two iron ions in the mixed-valence 1',6'-disubstituted biferrocene cation is emerging. The x-ray structures of diiodo-, dichloro-, and di-n-butylbiferrocenium triiodide salts have been determined. Two iron ions are on opposite sides of a planar fulvenide ligand. 1',6'-Dichlorobiferrocenium octaiodide(2-) transfers electrons slower than the Mossbauer time scale at 340 K. On the other hand, dibromo- and diiodobiferrocenium triiodide salts transfer electrons faster than the Mossbauer time scale not only at 298 K, but also all the way down to 4.2 K. Finally, mixed-valence triiodide salts of dibutyl- and dibenzylbiferrocenes each show one Fe('II) and one Fe('III) doublet in Mossbauer spectra measured belowed 200 K. As the temperature of these compounds is increased above 200 K the two doublets change to become eventually a single average doublet in the range of 250-275 K. The magnitude of the electronic interaction of the d-manifolds on the two iron ions in a given cation is probably not very different from one cation to another in the 1',6'-disubstituted mixed-valence biferrocenes. Furthermore, the vibronic coupling is also probably not changing very much throughout the series. It is our suggestion that the temperature dependence observed in the Mossbauer spectra of diethyl-, dipropyl-, dibutyl-, and dibenzylbiferrocenium triiodide salts results from a phase transition involving motion of the I(,3)('-) anion and perhaps the substituents on the cation. The anion dependence of electron transfer rate for the biferrocenium and dihalobiferrocenium cations and endothermic peaks seen in heat capacity and DSC data for biferrocenium triiodide further support the presence of phase transitions. A qualitative model is developed to explain the effect of the anion replacement. The importance of the 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.
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