The role of a terminal benzene ring on the mixed-valence state of a seriesof 1 ',1 '''-bis(n-phenylalkyl)-1,1 ''-biferrocenium triiodides

The relation between the mixed-valence state of the central Fe atoms and th e 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 d epend strongly on the methylene substituent in these salts. The 2-phenyleth yl and 4-phenylbutyl derivatives show valence detrapping with increasing te mperature, although their quadrupole splitting values are considerably diff erent from each other at lower temperatures. On the other hand, the 6-pheny lhexyl derivative shows the temperature-independent trapped-valence state. The initiation temperature of valence detrapping in 4-phenylbutyl derivativ e is lowered by changing the recrystallizing solvent from hexane to dichlor omethane, 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 v alues 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 s tates in the solid state. Both the cation and anion in the 2-phenylethyl de rivative are located on the center of symmetry, which is in good agreement with the Fe-57 Mossbauer spectrum results at room temperature. By compariso n of the present results with those of the corresponding dialkyl derivative s, the important role of the terminal benzene ring in the mixed-valence sta te is clarified. The role is based on intermolecular interaction via the be nzene ring. (C) 2000 Elsevier Science S.A. All rights reserved.