Synthesis, Redox Activity of Rigid Ferrocenyl Dendrimers, and Isolation of Robust Ferricinium and Class-II Mixed-Valence Dendrimers

The coupling reactions of ethynylferrocene with trihalogenoarenes do not lead to ethynylferrocenyl arenes that are soluble enough to form the basis of a suitable construction of stiff ferrocenylethynyl arene-cored dendrimers, which explains the previous lack of reports on stiff ferrocenyl dendrimers. However, rigid ferrocenyl-terminated dendrimers have been synthesized from 1,3,5-tribromo- and triiodobenzene through Sonogashira and Negishi reactions with 1,2,3,4,5-pentamethyl-1-ethylnylferrocene (1a), according to 12 connectivity. With compound 1a, the construction of a soluble dendrimer (10a) that contained 12 ethynylpentamethylferrocenyl termini was achieved. Stiff dendrimer 10a shows a single, reversible cyclic voltammetry (CV) wave (with adsorption), which disfavors the hopping heterogeneous electron-transfer mechanism that is postulated for redox-terminated dendrimers that contain flexible tethers. The selectivity of these Sonogashira reactions allows the synthesis of an arene-cored dendron (2c) that contains both ethynylferrocenyl and 1,2,3,4,5-pentamethyl-ferrocenylethynyl redox groups, thus leading to the construction of a dendrimer (7c) that contains both types of differently substituted ferrocenyl groups with two well-separated reversible CV waves. Upon selective oxidation, this mixed dendrimer (7c) leads to a class-II mixed-valence dendrimer, 7c[PF6]3, as shown by Mossbauer spectroscopy, whereas oxidation of the related fully pentamethylferrocenylated dendrimer (7a) leads to the all-ferricinium dendrimer, 7a[PF6]6.