Electron-, anion-, and proton-transfer processes associated with the redox chemistry of Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4) and its protonated form [Fe(η5-C5Ph5)((η6-C6H 5)C5Ph4H)]BF4

Alan M. Bond, Dirk A. Fiedler, Axel Lamprecht, Vanda Tedesco

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Voltammograms of microcrystals of Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4) and [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]BF4 mechanically attached to graphite or gold electrodes are well-defined when the electrode is placed in (70:30) water/acetonitrile (0.1 M electrolyte) media. The simplest processes at the electrode-solvent (electrolyte) interface are the chemically reversible oxidation of Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4), Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)(solid) + X-(solution) ⇌ [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X](solid) + e- when X- is the electrolyte anion (ClO4-, BF4-, Cl , or F-), and the chemically reversible reduction of [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]BF4, [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)][BF4] (solid) + e- ⇌ Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)(solid) + BF4-(solution), when BF4- is the electrolyte anion. Anion exchange between BF4-(solid) in [Fe-(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]BF4 and the electrolyte anion, X-(solution), is rapid so that the potentials of both processes are dependent on the electrolyte anion. Cyclic voltammograms scanned over a potential range encompassing both processes show that interconversion of [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]+ and Fe(η5-C6Ph5)((η6-C 6H5)C5Ph4) occurs when either [Fe-(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]+ or Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4) is initially present on the electrode surface until ultimately a voltammogram containing responses for both processes is achieved for a given electrolyte. Furthermore, the relative proportion of the two processes is a function of the "pH" of the solution phase, implying that the interfacial reaction Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)(solid) + H+(solution) + X-(solution) ⇌ [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)][X](solid) is chemically reversible. While interconversion of Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4) and [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]+ is slow, electrochemical oxidation of Fe(η5-C5Ph5)((η6-C 6H5)C5-Ph4) (solid) to [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X]2(solid) leads to very rapid formation of [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)][X](solid), possibly via the reaction scheme Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)(solid) + X-(solution) ⇌ [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X](solid) C6H5)C5Ph4)][X](solid) + X-(solution) ⇌ [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X]2(Solid) + e-; 2[Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X]2(solid) + 2H2O → 2[Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)][X](solid) + O2(solution) + 2H+(Solution) + 2X-(solution). However, the possible involvement of radical-based pathways in the conversion of [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)][X]2(solid) to [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]-[X](solid) cannot be excluded. An additional process, which is believed to be ligand based, is observed at a very positive potential. Electrospray mass spectrometric data confirm that [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)]+ is a product of oxidation of the parent compound and that conversion of both Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4)(solid) and its cation to [Fe(η5-C5Ph5)((η6-C 6H5)C5Ph4H)]X(solid) occurs, while the electrochemical quartz crystal microbalance data verify that anion transport across the electrode-solid-solvent (electrolyte) interface accompanies the electron- and proton-transfer reactions, thereby achieving charge neutralization.

Original languageEnglish
Pages (from-to)642-649
Number of pages8
Issue number4
Publication statusPublished - 15 Feb 1999

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