Molecular Wires using (Oligo)pyrroles as Connecting Units: An Electron Transfer Study

A series of (oligo)pyrroles featuring redox-active terminal ferrocenyl groups (Fc2-(cC4H2NPh)n (4, n = 1; 9, n = 2; 16, n = 3; 20, n = 4)) has been prepared using a Negishi C,C cross-coupling reaction protocol. The bi-, ter-, and quaterpyrrole wire moieties have been built up by C,C cross-coupling reactions of trimethylsilyl-protected pyrrole units in the presence of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 as precatalyst. The structural properties of the title compounds were investigated by spectroscopic means and single-crystal X-ray diffraction studies (9, 16, and 20). The influence of the increasing number of N-phenylpyrrole units on the electronic interaction between the iron centers was studied using electrochemistry (cyclic (CV) and square wave voltammetry (SWV)) as well as spectroelectrochemistry (in situ UV/vis/near-IR spectroscopy). With the exception of the diferrocenyl quaterpyrrole 20, the application of [NnBu4][B(C6F5)4] as electrolyte allows the discrete oxidation of the ferrocenyl termini (ΔE°′ = 450 mV (4), ΔE°′ = 320 mV (9), ΔE°′ = 165 mV (16)) in cyclic and square wave voltammograms. However, the iron centers of 20 were oxidized simultaneously, generating dicationic 202+. Additionally, one (9) or two (16 and 20) pyrrole-related well-defined reversible one-electron-redox processes were observed. The cyclic voltammetry data reveal that the splitting of the ferrocene-based redox couples, ΔE°′, decreases with increasing oligopyrrole chain length and, hence, a greater metal–metal distance. The trends in ΔE°′ with oligopyrrole structure also map to the electronic coupling between the ferrocene moieties, as estimated by spectroelectrochemical UV/vis/near-IR measurements. Despite the fact that there is no direct metal–metal interaction in diferrocenyl quaterpyrrole 20, a large absorption in the near-IR region is observed arising from photoinduced charge transfer from the oligopyrrole backbone to the redox-active ferrocenyl termini. These charge transfer absorptions have also been found in the dicationic oxidation state of the mono-(4), bi- (9), and terpyrroles (16). Within this series of diferrocenyl(oligo)pyrroles this CT band is shifted bathochromically with increasing chain length of the backbone motif.