Silver(I) complexes with long-chain diamines in non-aqueous solvents: Thermodynamic and modeling studies

The results of a potentiometric and calorimetric study on the complexation reactions of neutral N-donor ligands 1,4-diaminobutane (dab) and 1,5-diaminopentane (dap) with silver(I) in dimethylsulfoxide (DMSO) are reported. The investigation is extended for dab ligand to the aprotic solvent acetonitrile (AN), which presents quite different dielectric constant and donor number with respect to DMSO. In both solvents, polynuclear Ag2L22+ and Ag2L32+ species of high stability are formed, which inhibit the formation of mononuclear ones. The Ag2L22+ species are stabilized by the formation of large, stable non-chelate cyclic rings whose structures are described by DFT calculations. The overall enthalpy terms associated with the complex formation are strongly exothermic, whereas entropy values oppose the complex formation. An interesting feature is the little enthalpy and no significant or positive entropy variation associated with the successive formation of Ag2L32+. Also in this case, molecular modeling studies shed some light on their structure in solution, indicating that probably no changes of configuration around the metal ion occur. The stability constants and enthalpy values obtained for common Ag2L22+ species are slightly influenced by the chain length of the diamines, as confirmed also by a comparison with available data for the same species formed by the shorter 1,3-diamino propane (dapr) in DMSO.