Solvation and association of 3:1 electrolytes in N,N-dimethylformamide

A detailed study has been made of the solvation and ion association of the trifluoromethanesulfonate (Tf(-)) salts of aluminum(III), scandium(III), and lanthanum(III) in N,N-dimethylformamide (DMF) at 25 °C using dielectric relaxation spectroscopy over the frequency range of 0.1 ≲ ν/GHz ≤ 89. The spectra of all solutions exhibited either two (for ScTf3 and LaTf3) or three (for AlTf3) relaxation processes, a dominant mode centered at ∼15 GHz due to the solvent and one or (for AlTf3 solutions) two solute-related processes at lower frequencies (νmax ≲ 2 GHz). Effective solvation numbers, Zib, calculated from the solvent relaxation process indicated that all three cations were strongly solvated by DMF with Zib(0) values at infinite dilution in the order (Al(3+) ≈ Sc(3+) ≈ 10) < (La(3+) ≈ 13), consistent with at least partial formation of a second solvation shell around each cation. One solute-related mode for each set of salt solutions was assigned to the rotational diffusion of solvent-shared ion pairs (SIPs) of 1:1 stoicheometry; the additional slower process for AlTf3 solutions in DMF was attributed to the presence of double-solvent-separated IPs. The overall association constants at infinite dilution for the 1:1 IPs, KA°(MTf(2+)), were significant, but as expected from Debye-Hückel considerations, the KA values decreased rapidly with increasing solute concentration.