Synthesis, Characterization, Electrochemistry, and Computational Studies of Ferrocenyl-Substituted Siloles

Ferrocenylsiloles
of the type 2,5-Fc2-3,4-Ph2-cC4SiR2 (3a, R = Me; 3b, R = Ph) have been prepared by
reductive cyclization from diethynylsilanes, followed by ferrocenylation
using the Negishi C,C cross-coupling protocol with the silole ring
serving as either the vinyl halogenide species or as the zinc organic
component and the complementary functionality introduced on the ferrocenyl
moiety. The electrochemical behavior of these silacyclic-bridged bis­(ferrocenyl)
complexes was investigated by cyclic and square wave voltammetry,
and the nature of the redox products was studied by in situ UV–vis–near-IR
spectroelectrochemical measurements. 3a,b each undergo two sequential ferrocenyl-based redox processes, the
separation of which (ΔE°′ = ΔE2°′ – ΔE1°′ = 300 mV (3a), 280 mV (3b)) is in the range of structural similar systems such as
2,5-diferrocenyl-1-phenyl-1H-phosphole (280 mV) and
2,5-diferrocenylfuran (290 mV). Interestingly, the more electron rich
silole 3b, in comparison to 3a, shows a
modestly lower redox separation between the individual ferrocenyl
oxidation processes, which may be due to the capacity of this group
to shield the effect of an adjacent positive charge. An intervalence
charge transfer (IVCT) absorption was found in the in situ NIR spectra
of [3a]+ and [3b]+, the analysis of which is consistent with a moderate electronic
interaction between the iron atoms through the cis-diene-like fragment of the silole bridge, allowing their description
as Robin and Day class II mixed-valence systems. These conclusions
are supported by results from quantum chemical calculations, which
together with NMR studies of 3b, also reveal the likely
presence of a range of molecular conformations in solution.