Electron transfer reactions of hydrophobic metallocenes with aqueous redoxcouples at liquid-liquid interfaces. 1. Solvent, electrolyte, partitioning, and thermodynamic issues

Electron transfer reactions of the form, ML63+(w) + R-n-FER(o) --> ML62+(w) + RnFER+(o), where ML63+,2+(w) are hydrophilic aquo or amine transition me tal based redox couples and RnFER+,0(o) are hydrophobic alkyl-ferrocence re dox couples, can occur at the interface between aqueous (w) and immiscible organic (o) phases. The hydroxymethylferrocene(+,0) couple was used as an i nternal standard in order to compare the formal reduction potentials for a variety of aqueous couples with reduction potentials for the ferrocene coup les in organic solvents with dielectric constants ranging from 9 to 35. The ability of various electrolytes to provide adequate conductivity in aqueou s/organic phases, without inducing partitioning of alkylferricenium cations into the aqueous phase, was examined. The single electrolyte tetraethylamm onium tetrafluoroborate and the electrolyte consisting of potential determi ning ions tetrapropylammonium bromide/tetrapropylammonium tetraphenylborate were found to be generally suitable in these respects. Decamethylferroceni um ion was found to partition from organic to aqueous phases under certain conditions. In contrast, 1,1',3,3'-tetrakis(2-methyl-2-hexyl)-ferrocenium r emained in the organic phases. The rate of partitioning of alkylferricenium ions from benzyl cyanide thin films immobilized on carbon electrode surfac es was found to depend on the hydrophobicity of the cation and on the elect rolyte ions. Estimates of the interfacial potential difference, Delta(o)(w) phi, induced by the two favored electrolytes for several aqueous/organic so lvent interfaces were determined. These values of Delta(o)(w)phi combined w ith relative values of formal reduction potentials for aqueous and organic soluble redox couples can be used to estimate the driving force for a wide variety of electron transfer reactions at liquid/liquid interfaces. When th e tetrapropylammonium ion, a potential determining ion, was used as the ele ctrolyte, the value of Delta(o)(w)phi that was established conformed to the Nernst equation. The single electrolyte tetraethylammonium tetrafluorobora te established a value of Delta(o)(w)phi that was independent of salt conce ntration below 0.1 M.