Electron-transfer processes in metal-free tetraferrocenylporphyrin. Understanding internal interactions to access mixed-valence States potentially useful for quantum cellular automata

Abstract

Redox properties of H_2TFcP [TFcP^(2-) = 5,10,15,20-tetraferrocenylporphyrin^(2-)] were investigated using cyclic voltammetry, differential pulse voltammetry, and square-wave voltammetry methods in a large variety of solvents and electrolytes. When DMF, THF, and MeCN were used with TBAP as the supporting electrolyte, the first oxidation wave was assigned to a single four-electron oxidation process reflecting simultaneous oxidation of all iron(II) centers into iron(III) centers in H_2TFcP. When an o-DCB (1,2-dichlorobenzene)/TBAP combination was used in electrochemical experiments, four ferrocene substituents underwent two very diffuse, "two-electron" stepwise oxidations. The use of a weakly coordinating TFAB ([NBu_4][B(C_6F_5)_4]) electrolyte in o-DCB or DCM results in four single-electron oxidation processes for ferrocene substituents in which the first and second single-electron waves have a relatively large separation, while the second, third, and fourth oxidation processes are more closely spaced; similar results were observed when a DCM/TBAP system and an imidazolium cation-based ionic liquid ((bmim)Tf_2N = N-butyl-N'-methylimidazolium bis(trifluoromethanesulfonyl)imide) were used. Spectroelectrochemical oxidation of H_2TFcP in o-DCB or DCM with TFAB as the supporting electrolyte allowed for characterization of the mixed-valence [H_2TFcP]^+, [H_2TFcP]^(2+), and [H_2TFcP]^(3+) compounds by UV-vis spectroscopy in addition to the "all-Fe(III)" [H_2TFcP]^(4+). The chemical oxidation of H_2TFcP was tested using a variety of oxidants which resulted in formation of mixed-valence [H_2TFcP]^+ and [H_2TFcP]^(2+) as well as [H_2TFcP]^(4+), which were characterized by UV-vis-NIR, MCD, IR, Mossbauer, and XPS spectroscopy. The intervalence-charge-transfer bands observed in the near-IR region in [H_2TFcP]^+ and [H_2TFcP]^(2+) complexes were analyzed using Hush formalism and found to be of class II (in Robin-Day classification) character with localized ferrous and ferric centers. Class II behavior of [H_2TFcP]^+ and [H_2TFcP]^(2+) complexes was further confirmed by Mossbauer, IR, and XPS data.