Interaction of d10 metal ions with thioether ligands: a thermodynamic and theoretical study

Thermodynamic parameters of complex formation between d(10) metal ions, such as Zn2+, Cd2+, Hg2+ and Ag+, and the macrocyclic thioether 1,4,7-trithiacyclononane ([9]AneS3) or the monodentate diethylsulfide (Et2S), in acetonitrile (AN) at 298.15 K, were studied by a systematic methodology including potentiometry, calorimetry and polarography. [9]AneS3 is able to form complexes with all the target cations, Et2S only reacts with Hg2+ and Ag+. Mononuclear MLj (j = 1, 2) complexes are formed with all the metal ions investigated, where the affinity order is Hg2+ > Ag+ > Cd2+ approximate to Zn2+ when L = [9]AneS3 and Hg2+ > Ag+ when L = Et2S. Enthalpy and entropy values are generally negative, as a consequence of both metal ion interactions with neutral ligands, the reagents' loss of degrees of freedom and the release of solvating molecules. DFT calculations on the complexes formed with [9]AneS3 in vacuum and in AN are also carried out, to correlate experimental and theoretical thermodynamic values and to highlight the interplay between the direct metal-thioether interaction and the solvation effects. Trends obtained for the stability constants and enthalpies of the 1 : 1 and 1 : 2 complexes in solvent well reproduce the experimental ones for all the divalent metal ion complexes with [9]AneS3 and indicate the release of 3 AN molecules in the formation of each consecutive octahedral complex. In addition, calculated and experimental values for Ag+ complex formation in solution suggest that in AgL2 species [9] AneS3 ligands are not both tridentate.