Poly(biferrocenylethynyl)arene and bis(biferrocenyl)diynyl complexes and their redox chemistry

Orange linear bis-, star tris-, and dendritic tetra-biferrocenes linked by rigid ethynylaryl and diynyl spacers were synthesized through Sonogashira coupling and homocoupling reactions and oxidized to robust blue biferrocenium complexes. The proximity of the two ferrocenyl units to each other in the biferrocenyl units introduces electrostatic and electronic effects that are observed by cyclic voltammetry and are responsible for mixed-valence stabilization and localization. The use of the polyfluorinated electrolyte [n-Bu4N][BAr4F] {ArF = 3,5-bis(trifluoromethyl)phenyl} allows observing considerable enhancement of these effects and separation of electrochemical waves representing the two ferrocenyl groups of the biferrocenyl unit. The electrostatic effect is also selectively observed with the latter electrolyte between the two central ferrocenyl units of bis(biferrocenyl)diyne. Oxidation of all of these poly(biferrocenyl) complexes using a ferricinium salt yields blue, robust biferrocenium complexes. Their localized mixed-valent electronic structure was demonstrated at both Mössbauer and infrared time scales even with the counteranion (BAr4F) that provokes the maximum electrostatic effect and very much enhances the difference between the two oxidation potentials. Their near-infrared spectra show the intervalent charge transfer and are similar to those previously recorded for biferrocenium and derivatives, confirming the class-II mixed valence. The biferrocene units around the arene linker are completely electronically independent in the neutral and cationic complexes. In conclusion, from a practical standpoint, the easy oxidation of these stiff electrochromic nanosystems and the largely increased robustness of their oxidized form compared to ferricinium make their potential use as electrochromes considerably more attractive than that of simple ferrocene derivatives