Electronic communication in oligonuclear ferrocene complexes with anionic four-coordinate boron bridges

The di- and trinuclear ferrocene species Li[Fc-BPh2-Fc] (Li[9]) and Li-2[Fc-BPh2-fc-BPh2-Fc] (Li-2[10]) have been investigated with regard to their electrochemical properties and the degree of intervalence charge-transfer after partial oxidation. Li[9] shows two distinct one-electron redox waves for its chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E-1/2 = -0.38 V, -0.64 V; vs. FcH/FcH(+)). The corresponding values of Li-2[10] are E-1/2 = -0.45 V (two-electron process) and -1.18 V. All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray crystallography on the mixed-valent (Fe2FeIII)-Fe-II complex Li(12-c-4)(2)[10] reveals the centroid-centroid distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 angstrom) to be significantly smaller than in the central 1,1'-ferrocenediyl fragment (3.420 angstrom). This points towards a charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being in the Fe-III state. Mossbauer spectroscopic measurements on Li(12-c-4)(2)[10] lend further support to this interpretation. Spectroelectrochemical measurements on Li[9] and Li-2[10] in the wavelength range between 300-2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction between the individual Fe atoms in Li[9] and Li-2[10] occurs via a through-space pathway and/or is electrostatic in nature.