The role of a terminal benzene ring on the mixed-valence state of a series of 1′,1‴-bis( n-phenylalkyl)-1,1″-biferrocenium triiodides

Abstract

The relation between the mixed-valence state of the central Fe atoms and the crystal structure is discussed in a series of 1′,1‴-bis( n-phenylalkyl)-1,1″-biferrocenium triiodides ( n=2, 3, 4, 6). The mixed-valence states depend strongly on the methylene substituent in these salts. The 2-phenylethyl and 4-phenylbutyl derivatives show valence detrapping with increasing temperature, although their quadrupole splitting values are considerably different from each other at lower temperatures. On the other hand, the 6-phenylhexyl derivative shows the temperature-independent trapped-valence state. The initiation temperature of valence detrapping in 4-phenylbutyl derivative is lowered by changing the recrystallizing solvent from hexane to dichloromethane, accompanied by a change in the powder X-ray diffraction pattern. The temperature dependences of the quadrupole splitting values in the salts under investigation can be explained using constant quadrupole splitting values both for typical ferrocene- and ferrocenium-like doublets and only by changing the energy difference between the Fe II–Fe III and Fe III–Fe II states in the solid state. Both the cation and anion in the 2-phenylethyl derivative are located on the center of symmetry, which is in good agreement with the 57Fe Mössbauer spectrum results at room temperature. By comparison of the present results with those of the corresponding dialkyl derivatives, the important role of the terminal benzene ring in the mixed-valence state is clarified. The role is based on intermolecular interaction via the benzene ring.