Variable pressure NMR and spectrophotometric study of lanthanide solvation in DMF

The equilibrium [Ln(DMF)8]3+ + DMF ⇄ [Ln(DMF)9]3+ has been studied in solutions of lanthanide perchlorates. For Nd the following thermodynamic parameters were obtained from spectrophotometric studies: ΔH = −14.9 ± 13. kJ mol−1, ΔS = −69.1 ± 4.2 J K−1 mol−1 and ΔV = −9.8 ± 1.1 cm3 mol−1. The NMR study of Ln = CeNd and TbYb shows that the proportion of the nine coordinate solvates rapidly becomes insignificant across the lanthanide series. Characteristics 17O, 13C and 1H NMR shifts for [Ln(DMF)8]3+ are also reported. Kinetic parameters for the exchange of DMF on [Ln(DMF)8]3+ (Ln = TbYb) have been determined in neat DMF, by variable temperature and pressure 1H and 13C-NMR at 1.4, 4.7 and 8.5 Tesla. For the high field high pressure experiments, a high resolution multinuclear probe fitting into the tubular wide-bore 4.7 Tesla superconducting magnet was built. The probe works between −60 to +130 °C and has been used to 250 MPa. The spectral resolution measured by 1H-NMR is about 1 Hz, that is 5 × 10−9. For the DMF exchange on Tb, the activation parameters, ΔH* kJ mol−1), ΔS* (J mol−1 K−1 and ΔV* (cm3 mol−1, are respectively, 14.1 ± 0.4, −58 ± 2 and +5.2 ± 0.2 whilst for Yb the corresponding values are 39.3 ± 0.6, +40 +- 3 and +11.8 ± 0.4. The observed systematic variations in activation parameters from Tb to Yb are interpreted in terms of a mechanistic crossover at Er, Kinetic rate law determinations in CD3NO2 diluent indicate that an interchange mechanisms operates for Tb (kos = 0.5 at 231 K) whereas a D mechanism is operative for Yb.