Ion–solvent interactions in mixtures of N-methyl-2-pyrrolidinone and N-methyl-2-thiopyrrolidinone

Abstract

Electrochemical measurements of hard, borderline and soft cations such as Li⁺, Na⁺, K⁺, Tl⁺, Cu⁺, Ag⁺ and Cd²⁺ as well as studies on the redox behaviour of oxygen, perylene and tetrabutylammonium hexacyanoferrate have been carried out in mixtures of the hard oxygen donor solvent N-methyl-2-pyrrolidinone and the soft sulphur donor solvent N-methyl-2-thiopyrrolidinone. Soft cations were found to be strongly stabilized by N-methyl-2-thiopyrrolidinone, and significant changes in the redox potentials to more positive values could be observed only in solutions rich in N-methyl-2-pyrrolidinone. The opposite effect was observed for hard cations. These cations interacted strongly with N-methyl-2-pyrrolidinone, and the redox potentials remained more or less constant until a considerable excess of N-methyl-2-thiopyrrolidinone appeared. The behaviour of the redox systems, which included anionic species, namely O₂/O^–₂ and Fe(CN)^3–₆/Fe(CN)^4–₆ was only slightly affected by changes in the solvent composition. Gibbs energies of transfer from N-methyl-2-pyrrolidinone into the solvent mixtures for Li⁺, Na⁺, K⁺, Cu⁺, Ag⁺, Tl⁺ and Cd²⁺ were calculated from the electrochemical data on the basis of the bis(biphenyl)chromium uptake. In addition to the studies on the changes in redox potentials, the solvent-induced shifts of the visible spectra of the solvatochromic dyes acetylacetonato(tetramethylethylenediamine)-copper(II) perchlorate, bis(cyano)bis(1,10-phenanthroline)iron(II), bis(cyano)bis(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridino)phenoxide were investigated. The results are interpreted on the basis of changes in the Lewis-type donor and acceptor properties of the solvent mixtures including the principle of hard and soft acids and bases.