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

3,4-Diiodo-1-(triisopropylsilyl)-1<i>H</i>-pyrrole (<b>1</b>), 3,4-diferrocenyl-1-(triisopropylsilyl)-1<i>H</i>-pyrrole (<b>2</b>), and 3,4-diferrocenyl-1<i>H</i>-pyrrole (<b>3</b>) were prepared and characterized using spectroscopic methods and X-ray crystallography. UV–vis spectra of <b>2</b> and <b>3</b> 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 <b>2</b> and <b>3</b> were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical approaches. Ferrocene-centered oxidation processes in <b>2</b> and <b>3</b> 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 <b>2</b> and <b>3</b> allowed us to obtain spectroscopic signatures of the mixed-valence [<b>2</b>]⁺ and [<b>3</b>]⁺ cations. Hush analysis of the intervalence charge-transfer band in [<b>2</b>]⁺ and [<b>3</b>]⁺ 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