Electron-, anion-, and proton-transfer processes associated with the redoxchemistry of Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4) and its protonated form [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)]BF4 at microcrystal-electrode-solvent (Electrolyte) interfaces

Voltammograms of microcrystals of Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4) and [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)]BF4 mechanically attached to graphit e or gold electrodes are well-defined when the electrode is placed in (70:3 0) water/acetonitrile (0.1 M electrolyte) media. The simplest processes at the electrode-solvent (electrolyte) interface are the chemically reversible oxidation of Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4), Fe(eta(5)-C5Ph5)((eta(6 )-C6H5)C5Ph4)((solid)) + X-(solution(-)) reversible arrow [Fe(eta(5)-C5Ph5) ((eta(6)-C6H5)C5Ph4)][X]((solid)) + e(-) when X- is the electrolyte anion ( ClO4-, BF4-, Cl-, or F-), and the chemically reversible reduction of [Fe(et a(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H]BF4, [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H) ][BF4]((solid)) + e(-) reversible arrow Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4 H)((solid)) + BF4(solution-), when BF4- is the electrolyte anion. Anion exc hange between BF4(solid-) in [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)]BF4 and the electrolyte anion, X-(solution(-)), is rapid so that the potentials of both processes are dependent on the electrolyte anion. Cyclic voltammogram s scanned over a potential range encompassing both processes show that inte rconversion of [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)](+) and Fe(eta(5)-C5P h5)((eta(6)-C6H5)C5Ph4) occurs when either [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C 5Ph4H)](+) or Fe(eta(5)-C5Ph5)((eta(6)-C6H5)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 re lative proportion of the two processes is a function of the "pH" of the sol ution phase, implying that the interfacial reaction Fe(eta(5)-C5Ph5)((eta(6 )-C6H5)C5Ph4) ((solid)) + H-(solution(+)) + X-(solution(-)) reversible arro w [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)][X]((solid)) is chemically reversi ble. While interconversion of Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4) and [Fe(eta(5 )-C5Ph5)((eta(6)-C6H5)C5Ph4H)](+) is slow, electrochemical oxidation of Fe( eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4) ((solid)) to [Fe(eta(5)-C5Ph5)((eta(6)-C6 H5)C5Ph4)][X](2(solid)) leads to very rapid formation of [Fe(eta(5)-C5Ph5)( (eta(6)-C6H5)C5Ph4H)][X]((solid)), possibly via the reaction scheme Fe(eta( 5)-C5Ph5)((eta(6)-C6H5)C5Ph4)((solid)) + X-(solution(-)) reversible arrow [ Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4)][X]((solid)) + e(-); [Fe(eta(5)-C5Ph5) ((eta(6)-C6H5)C5Ph4)][X]((solid)) + X-solution(-) reversible arrow [Fe(eta( 5)-C5Ph5)((eta(6)-C6H5)C5Ph4)][X](2(solid)) + e(-); 2[Fe(eta(5)-C5Ph5)((eta (6)-C6H5)C5Ph4)][X](2(solid)) + 2H(2)O --> 2[Fe(eta(5)-C5Ph5)((eta(6)-C6H5) C5Ph4H)][X]((solid)) + O-2(solution) + 2H((solution)(+)) + 2X((solution)(-) ). However, the possible involvement of radical-based pathways in the conve rsion of [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4)][X](2(solid)) to [Fe(eta(5)- C5Ph5)((eta(6)-C6Ph5)C5Ph4H)][X]((solid)) cannot be excluded. An additional process, which is believed to be ligand based, is observed at a very posit ive potential. Electrospray mass spectrometric data confirm that [Fe(eta(5) -C5Ph5)((eta(6)-C6H5)C5Ph4)](+) is a product of oxidation of the parent com pound and that conversion of both Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4)((sol id)) and its cation to [Fe(eta(5)-C5Ph5)((eta(6)-C6H5)C5Ph4H)]X-(solid) occ urs, 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.