Electron-Transfer Processes in 3,4-Diferrocenylpyrroles: Insight into a Missing Piece of the Polyferrocenyl-Containing Pyrroles Family

3,4-Diiodo-1-(triisopropylsilyl)-1H-pyrrole (1), 3,4-diferrocenyl-1-(triisopropylsilyl)-1H-pyrrole (2), and 3,4-diferrocenyl-1H-pyrrole (3) were prepared and characterized
using spectroscopic
methods and X-ray crystallography. UV–vis spectra of 2 and 3 were correlated with their density functional
theory (DFT)-calculated electronic structures as well as theoretically
predicted by the time-dependent (TD) DFT-calculations vertical excitation
energies. Redox properties of 2 and 3 were
investigated using cyclic voltammetry, differential pulse voltammetry,
and spectroelectrochemical approaches. Ferrocene-centered oxidation
processes in 2 and 3 were found to be separated
by ∼180 and ∼300 mV in DCM/TBAP and DCM/(NBu4)­[B­(C6F5)4] systems, respectively.
Stepwise spectroelectrochemical oxidation of 2 and 3 allowed us to obtain spectroscopic signatures of the mixed-valence
[2]+ and [3]+ cations.
Hush analysis of the intervalence charge-transfer band in [2]+ and [3]+ is suggestive of class
II (in Robin and Day classification) mixed-valence behavior. Electronic
structures of neutral and spin-localized/delocalized single-electron
oxidized mixed-valence cations of 2,5-di-, 3,4-di-, and 2,3,4,5-tetraferrocenylpyrroles
were investigated by DFT calculations to resolve current uncertainties
regarding the first oxidation process of tetraferrocenylpyrrole.