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

Redox properties of H2TFcP [TFcP2- = 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 H2TFcP. 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 ([NBu4][B(C6F5)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)Tf2N ) N-butyl-N′-methylimidazolium bis(trifluoromethanesulfonyl)imide) were used. Spectroelectrochemical oxidation of H2TFcP in o-DCB or DCM with TFAB as the supporting electrolyte allowed for characterization of the mixed-valence [H2TFcP]+, [H2 TFcP] 2+, and [H2TFcP]3+ compounds by UV-vis spectroscopy in addition to the "all-FeIII" [H 2TFcP]4+. The chemical oxidation of H2TFcP was tested using a variety of oxidants which resulted in formation of mixed-valence [H2TFcP]+ and [H2TFcP]2+ as well as [H2TFcP]4+, which were characterized by UV-vis-NIR, MCD, IR, Mössbauer, and XPS spectroscopy. The intervalence-charge-transfer bands observed in the near-IR region in [H2TFcP]+ 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 [H2TFcP]+ and [H2TFcP]2+ complexes was further confirmed by Mössbauer, IR, and XPS data. © 2009 American Chemical Society.