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

Abstract

The di- and trinuclear ferrocene species Li[Fc-BPh₂-Fc] (Li[9]) and Li₂[Fc-BPh₂-fc-BPh₂-Fc] (Li₂[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 Li₂[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 Fe^II₂Fe^III 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 Å) to be significantly smaller than in the central 1,1′-ferrocenediyl fragment (3.420 Å). 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. Mößbauer spectroscopic measurements on Li(12-c-4)₂[10] lend further support to this interpretation. Spectroelectrochemical measurements on Li[9] and Li₂[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 Li₂[10] occurs via a through-space pathway and/or is electrostatic in nature.