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/(NBu ₄)[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.