Molecular Wires using (Oligo)pyrroles as Connecting Units: An Electron Transfer Study
dataset
posted on 2016-02-18, 13:32 authored by Ulrike Pfaff, Alexander Hildebrandt, Dieter Schaarschmidt, Tobias Rüffer, Paul J. Low, Heinrich LangA 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.
