Solvent effect on heavy metal coordination with thioether ligands: A thermodynamic and theoretical study

Abstract

The thermodynamic parameters for the complexation between the monodentate diethylsulfide (Et2S), the macrocyclic thioethers 1,4,7-trithiacyclononane ([9]aneS3) 1,4,7,10-tetrathiacyclododecane ([12]aneS4) and 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) have been determined in dimethylsulfoxide (DMSO) with d10 metal ions such as Hg2+, Ag+, Cd2+and Zn2+.Mononuclear MLj (j=1, 2) complexes are formed by all ligands with Hg2+ and Ag+ with associated negative enthalpy and entropy changes. Differently from what previously found in acetonitrile (AN), no reaction occurs with Zn2+ and Cd2+.The stabilities are lower in DMSO and the selectivity, much higher for Hg2+ than Ag+ in AN, is leveled in DMSO, being higher only when [9]aneS3 is concerned (ΔlogK1,Hg→Ag=8.9 and 0.9 for [9]aneS3 in AN and DMSO, respectively). Density Functional Theory (DFT) calculations are used to interpret at molecular level the subtle interplay between the intrinsic affinity of a given metal ion for a thioether and solvation in different media. The reliability of this approach is demonstrated by the good correlation obtained for the calculated and theoretical solvation energy of the metal ions. Theoretical results clearly indicate that: (i) the medium (solvent) strongly modulates the relative selectivity of thioethers towards different cations, i.e. Hg2+ and Ag+; (ii) Zn2+ complexes are not formed in DMSO mostly because of the strong solvation of the ion; (iii) for Cd2+ the less favorable pure interaction with the ligands is most detrimental to complex formation.