Electrochemical Generation of Soluble and Reactive Cadmium, Lead, and Thallium Cations in Noncoordinating Solvents: Relative Strengths of Perchlorate, Tetrafluoroborate, and Hexafluorophosphate Ligation in Dichloromethane and Benzene

Article abstract of DOI:10.1021/ja00224a010

Electrochemical oxidation of metal amalgam electrodes in noncoordinating solvents generates soluble forms of highly active metal ions at the electrode surface that can form very strong complexes with the tetrafluoroborate and perchlorate anions.Oxidation at Cd, Pb, and Tl dropping mercury amalgam electrodes in a solution containing dichloromethane and either tetrabutylammonium hexafluorophosphate, , tetrabutylammonium tetrafluoroborate, , or tetrabutylammonium perchlorate, , occurs reversibly, thereby allowing thermodynamic data to be obtained on the nature of the complex formed.The ^- ligation is considerably weaker than that of ^- or ^-.Data obtained in dichloromethane with hexafluorophosphate, as a reference electrolyte allowed the following complexes with their equilibrium constants to be identified: ^2-, log β₄=9.1; ^-, log β₃=7.3; -, log β₃=8.3; Pb(BF4)2, log β₂=7.5; Tl(ClO4), log β₁=3.3; Tl(BF4), log β₁=2.9.The equilibrium constants for these complexes are larger then those obtained in aqueous media for many classical ligands.In benzene, which is of lower dielectric constant and is less polar than dichoromethane, the half-wave potential for the oxidation of the cadmium amalgam electrode is approximately 700 mV more negative with pechlorate than with hexafluorophosphate as the electrolyte anion.In contrast, the difference is only 200 mV in dichloromethane.This unprecendented difference may be attributed to the weaker coordination of benzene and the consequent amplification of the differences in strength of perchlorate and hexafluorophosphate ligation.Consequently, a method of preparing highly activated and previously unknown forms of soluble metal ions is available in solvents such as chlorinated and aromatic hydrocarbons. This feature is further illustrated by the large negative shift in half-wave potential for the metal oxidations observed after the coordinating solvent dimethyl sulfoxide is added to dichloromethane (0.2 M ) solutions.Controlled-potential electrolysis experiments at a mercury amalgam pool in dichloromethane lead to the formation of the expected nonsolvated insoluble salts, demonstrating that the kinetics of precipitation are slower then the polarographic time scale.Concepts developed on this work provide prospects for new forms of mechanistic, thermodynamic and synthetic metal ion chemistry.