Solvation of copper(II) sulfate in binary water/N,N-dimethylformamide mixtures:: From the solution to the gas phase

The solvation of copper(II) sulfate in binary mixtures of water and N,N-dimethylformamide (DMF) is studied by a combined approach using electrochemical studies in solution and a mass spectrometric assay of the solvated ions formed from these solutions upon electrospray ionization (EST). In the condensed phase, the limiting transference numbers (t(+/-)degrees) and the apparent ion association constants (K-A's) of CuSO4 have been determined in water/DMF solutions at 20 degrees C. The t(+)degrees values decrease with increasing DMF content, demonstrating a gradual solvation of Cu2+ by DMF molecules. The association constants indicate that aggregation becomes more pronounced as the DMF content increases. In order to achieve complementary insight, the intrinsic interactions among the ions and solvent molecules are investigated in gas-phase experiments of the CuSO4/water/DMF system using EST mass spectrometry. Under the conditions used, the dications [Cu(DMF)(n)](2+) (n = 3-6), [Cu-2(DMF)(n)SO4](2+) (n = 2-7), and [Cu-3(DMF)(n)(SO4)(2)](2+) (n = 2-7), and the monocations [Cu(OH)(DMF)(n)](+), [Cu(DMF)(n)(HSO4)](+) (both, n = 1-3), and [Cu(DMF)(n)](+) (n = 1, 2), are formed as the leading copper-containing cations. Likewise, polynuclear copper clusters observed in the anion ESI spectra support partial aggregation occurring in solution. The gas-phase studies clearly support the conclusions that (i) DMF is a highly preferred ligand for Cull in comparison to water and that (ii) DMF supports ion association for which the mass spectrometric data suggest the formation of polynuclear copper clusters.