Effects of Environment on Intramolecular Electron Transfer in Mixed-Valence 1',l'''-Dinaphthylmethyl(biferrocenium) Triiodide: Structural and ~(57)Fe Mossbauer Characteristics

摘要

Electron transfer in chemical and biological systems has received considerable attention in the last few years. Recent studies of electron transfer in mixed-valence biferrocenium complexes 1-6 revealed that the environment surrounding a cation is perhaps the most important factor in determining the rate of intramolecular electron transfer. Complexes 1-6 give unusual temperature-dependent Mossbauer spectra. At temperatures below 77 K they each show two doublets, one for the Fe~(II) and one for the Fe~(III) site (electron-transfer rate <10~7 s~(-1)). In each case the two doublets move together upon increasing temperature and eventually become a single "average-valence" doublet. Hendrickson et al. suggested that the temperature dependence of the Mossbauer spectrum is due to the onset of lattice dynamics associated with the triiodide counterions and alkyl substituents. At low temperatures (< 100 K) all parts of the crystal lattice of the complex are static. As the temperature is increased, the thermalenergy formed could be sufficient to trigger a cooperative phase transition in the crystal lattice. A crystallographic phase change was not observed in 1-6; we believe that the transition may be described in terms of a gradual (second-order) change withtemperature.