Organometallic Electrochemistry Based on Electrolytes Containing Weakly-Coordinating Fluoroarylborate Anions

Abstract

Electrochemistry is a powerful tool for the study of oxidative electron-transfer reactions (anodic processes). Since the 1960s, the electrolytes of choice for nonaqueous electrochemistry were relatively small (heptaatomic or smaller) inorganic anions, such as perchlorate, tetrafluoroborate, or hexafluorophosphate. Owing to the similar size-to-charge ratios of these "traditional" anions, structural alterations of the electrolyte anion are not particularly valuable in effecting changes in the corresponding redox reactions. Systematic variations of supporting electrolytes were largely restricted to cathodic processes, in which interactions of anions produced in the reactions are altered by changes in electrolyte cations. A typical ladder involves going from a weakly ion-pairing tetraalkylammonium cation, [N(C(n)H(2n+1))(4)](+), with n > or = 4, to more strongly ion-pairing counterparts with n < 4, and culminating in very strongly ion-pairing alkali metal ions. A new generation of supporting electrolyte salts that incorporate a weakly coordinating anion (WCA) expands anodic applications by providing a dramatically different medium in which to generate positively charged electrolysis products. A chain of electrolyte anions is now available for the control of anodic reactions, beginning with weakly ion-pairing WCAs, progressing through the traditional anions, and culminating in halide ions. Although the electrochemical properties of a number of different WCAs have been reported, the most systematic work involves fluoro- or trifluoromethyl-substituted tetraphenylborate anions (fluoroarylborate anions). In this Account, we focus on tetrakis(perfluorophenyl)borate, [B(C(6)F(5))(4)](-), which has a significantly more positive anodic window than tetrakis[(3,5-bis(trifluoromethyl)phenyl)]borate, [BArF(24)](-), making it suitable in a larger range of anodic oxidations. These WCAs also have a characteristic of specific importance to organometallic redox processes. Many electron-deficient organometallic compounds are subject to nucleophilic attack by the traditional family of electrolyte anions. With a view to testing the scope of the much less nucleophililic WCAs in providing a benign electrolyte anion for the generation of organometallic cation radicals, we carried out a series of studies on transition metal sandwich and half-sandwich compounds. The model compounds were chosen both for their fundamental importance and because their radical cations had been neither isolated nor spectrally characterized, despite many previous electrochemical investigations with traditional anions. The oxidation of prototypical organometallic compounds, such as the sandwich-structured ruthenocene and the piano-stool structured Cr(eta(6)-C(6)H(6))(CO)(3), Mn(eta(5)-C(5)H(5))(CO)(3), Re(eta(5)-C(5)H(5))(CO)(3), and Co(eta(5)-C(5)H(5))(CO)(2), gave the first definitive in situ characterization of their radical cations. In several cases, the kinetic stabilization of the anodic products allowed the identification of dimers or unique dimer radicals having weak metal-metal bonds and provided new preparative options for organometallic systems. In terms of thermodynamic effects, the lower ion-pairing abilities of WCAs and their good solubility in a broad range of solvents, including those of lower polarity, permitted a systematic study that yielded an integrated model of how to use solvent-electrolyte combinations to manipulate the E(1/2) differences of compounds undergoing multiple electron-transfer reactions. Although the efficacy of WCA-based electrolytes in organometallic anodic chemistry is now established, WCAs might further expand applications of organic redox chemistry. Other WCAs, including those derived from carboranes and fluorinated alkoxyaluminates, merit additional studies.