Solvent and structural effects on silver(I) complex formation: thermodynamics and modeling

The results of a potentiometric and calorimetric study on the complexation of silver(I) with neutral N-donors 1,2-diaminoethane (EN), 1,3-diaminopropane (DAP), 1,4,7-triazaheptane (DIEN), 1,5,9-triazanonane (DPT) and 1,4,7,10-tetraazadecane (TRIEN) in acetonitrile (AN) are reported. The overall enthalpy terms associated with the complex formation are strongly exothermic, whereas the entropy values oppose the complex formation. Polynuclear species are formed by DAP and DPT, the ligands where the N atoms are separated by a longest carbon chain, but not by EN and DIEN. This also occurs in dimethyl sulfoxide (DMSO) where data for the same amines are available, and a comparison can be made. The Ag3L23+ species formed by DPT is stabilized by the formation of large rings, where six Ag–N bonds are formed in an organized, compact, structure, which is proposed on the basis of density functional theory (DFT) calculations. A remarkable, interesting, aspect emerged in this study is the similarity of formation constants for the same complexes in DMSO and AN, unexpected on the basis of the quite large difference between the donor properties of AN and DMSO. A careful analysis of the thermodynamic transfer parameters from AN to DMSO was made on Ag(I)–EN system for all the species participating in the reactions. The results allow to conclude that the solvation of the complexes is mainly responsible for this unexpected behavior. The comparison of the thermodynamic functions available in dimethylformamide (DMF) and propylenecarbonate (PC) indicates the following stability order of Ag-EN and -DIEN complexes: DMSO ≈ AN < DMF < PC. The trend in DMF and PC, as compared to AN, seems to reflect the prevailing importance of the silver(I) ion solvation on the complexation thermodynamics.