Manipulating the Electrolyte Medium to Favor Either One-Electron or Two-Electron Oxidation Pathways for (Fulvalendiyl)dirhodium Complexes

Abstract

The anodic oxidations of three compounds having two Rh-containing moieties linked by a fulvalendiyl (Fv) dianion have been studied in varying electrolyte media. Following the integrated approach to manipulation of solvents and supporting electrolytes (Barrie ´re, F.; Geiger, W. E. J. Am. Chem. Soc., in press),) of the successive one-electron oxidations of the neutral compounds to the corresponding dications were altered to either favor or disfavor the disproportionation of the mixed-valent intermediate. In the cases of Rh 2 Fv(CO) 4 (1) and Rh 2 Fv(CO) 2 (µ-dppm) (2) [Fv ) C 10 H 8 , dppm ) bis(diphenylphosphino)methane], replacing the [PF 6 ]supporting electrolyte anion with [B(C 6 F 5 )]in CH 2 Cl 2 changed the voltammograms from those of a single two-electron wave (having "inverted" E 1/2 potentials) to those of two distinct "normal" one-electron waves. In the case of Rh 2 Fv-(COD) 2 (3, COD ) C 8 H 12 ), both the solvent and supporting electrolyte were changed to manipulate ∆E 1/2 between normal and inverted values. Changes of up to 330 mV in ∆E 1/2 were observed, resulting in modifications of over 10 5 in the disproportionation equilibrium constant, K disp , of the mixed-valent intermediate 3 + . The thermodynamic stabilization of 1 + and 2 + allowed for their IR characterization and confirmed the previously postulated formation of Rh-Rh bonds in the cation radicals. An integrated approach to medium effects is shown to be a powerful method for manipulating the equilibrium concentrations of the various redox states that make up a multiple-electron-transfer process.

Tetraalkylammonium Salts of Weakly Coordinating Aluminates: Ionic Liquids, Materials for Electrochemical Applications and Useful Compounds for Anion Investigation

“…À anion, [11,12] but the synthesis of this fluorinated borate is much more complicated and also more expensive. They provide also a good alternative to the widely used [PF 6 ] À salts in non-polar media.…”

“…[7] [ 4 ] À have been reported both to be very stable and to effectively solubilize the positively charged species formed in anodic processes. [8][9][10][11][12] However, electrochemical investigations in very non polar solvents like toluene or benzene are limited to only very few anions, for example, [CB 11 Me 12 ] À . [13,14] Unfortunately the synthesis of this anion is very complicated, which makes it impossible to be widely used as a WCA for electrochemical applications.…”

“…[8][9][10][11][12] Using the difficult to prepare very lipophilic [B 11 CMe 12 ] À anion, [13] even cyclovoltammetric measurements in benzene may be performed. [14] Another alternative to circumvent the solvent problem are tetralkylammonium salts with long alkyl chains which were used in ultramicroelectrode experiments without any supporting electrolyte.…”

Spectroscopic and Electrochemical Evaluation of Salt Effects on Electron‐Transfer Equilibria between Donor/Acceptor and Ion‐Radical Pairs in Organic Solvents

“…This deficiency is apparently related to the limited range of (donor/acceptor) redox potentials appropriate for the experimental measurements of both reactants and products in Equation (1), [11] as well as to the high reactivity of the radical species D + C and A À C. [10] Thus, in contrast to the plentiful reports on electrolyte modulation of acid/base equilibria, the studies of one-electron transfers with donor/acceptor and ion-radical pairs have mostly concentrated on thermal reaction kinetics and product yields, [1][2][3][4][5][6] dynamics of photochemically generated ion-radical pairs, [7,8] or redox-potential measurements. [12][13][14] For example, Fukuzumi and co-workers reported considerable changes in the redox-reaction rates of many donor/acceptor systems upon additions of salts (especially those containing a scandium cation). [6,13] The researchers related such effects mostly to the tuning of the acceptors' reduction potentials by the electrolyte (due to anion-radical coordination either with metal ions or with hydrogen-bonding alkylammonium).…”

“…In another recent publication, Berriere and Geiger described drastic effects of electrolytes on the successive redox potentials of multielectron complexes in organic solvents (and therefore, their comproportionation equilibria). [14] However, conventional electrochemical studies require certain concentrations of supporting electrolyte and neglect direct cation-radical/anion-radical and donor/acceptor interactions, which hinder their application in the evaluation of salt effects in electron-transfer equilibria.…”