Cubic Octasilsesquioxanes, Cyclotetrasiloxanes, and Disiloxanes Maximally Functionalized with Silicon-Bridged Interacting Triferrocenyl Units

Redox-active, highly symmetrical cubic octasilsesquioxanes
(OS)
peripherally decorated with 24 ferrocenyl units, linked in threes
around the periphery of a cubic cage, namely, [Fc3Si­(CH2)2Me2SiO]8Si8O12 (6) and [Fc3Si­(CH2)2]8Si8O12 (7)
(Fc = (η5-C5H4)­Fe­(η5-C5H5)), have been synthesized. Such
integrally ferrocenyl-functionalized cubic macromolecules 6 and 7, as well as the related small-molecule models
hexaferrocenyldisiloxane [Fc3Si­(CH2)2Me2Si]2O (4) and dodecaferrocenyl
cyclotetrasiloxane [Fc3Si­(CH2)2MeSiO]4 (5), have been prepared by covalently linking,
via Karstedt’s-catalyzed hydrosilylation, triferrocenylvinylsilane
(CH2CH)­Si­(Fc)3 (3) around
the surface of octasilsesquioxane cages T8(OSiMe2H)8 and T8H8 and linear [Me2SiH]2O and cyclic [MeSiHO]4 siloxane
scaffolds, respectively. All new polyferrocenyl oligosiloxanes have
been thoroughly characterized using a combination of elemental analysis,
multinuclear (1H, 13C, 29Si) NMR
spectroscopy, FT-IR, and MALDI-TOF mass spectrometry. The molecular
structure of disiloxane 4, in the solid state, has been
determined by single-crystal X-ray analysis. Hexametallic 4 shows a bent arrangement of the ferrocenyl-substituted disiloxane
linkage (Si–O–Si angle of 147.6(5)°). Polyferrocenyl-OS 6 and 7 show good thermal stability and form
iron-containing ceramics when pyrolyzed under nitrogen. The electrochemical
behavior of polyferrocenyl OS and model linear and cyclic siloxanes
has been examined by cyclic and square wave voltammetries, in dichloromethane
solution using PF6– and B­(C6F5)4– as supporting electrolyte
anions of different coordinating ability. The novel maximally ferrocenyl-functionalized
oligosiloxanes exhibit a three-wave redox pattern, suggesting appreciable
electronic interactions between the silicon-bridged triferrocenyl
moieties as they are successively oxidized. OS 6 and 7 undergo remarkable oxidative precipitation in CH2Cl2/n-NBu4PF6 and
are able to form stable electroactive films on platinum electrode
surfaces. They are the first redox-active OS showing significant electronic
interactions between metal sites on the cage surface.