NMR studies of mixed-ligand lanthanide (III) complexes. Peculiarities of molecular structure, dynamics and paramagnetic properties for cerium subgroup chelates with crown ethers

The molecular structure, dynamics and paramagnetic properties of the complex cations [Ln(ptfa)(2) (18-crown-6)](+) in deuterated toluene were studied for Ln = La, Ce, Pr and Nd. The activation enthalpy values of 68 +/- 5, 55 +/- 15 and 60 +/- 13 kJ mol(-1) for the 18-crown-6 conformational inversion processes for the complex cations of Ce, Pr and Nd, respectively, were obtained. Quantitative investigation of the lanthanide-induced chemical shifts shows a monotonic change of a spatial structure and magnetic susceptibility in comparison with the Bleaney predicted dependence. The free energy of molecular inversion activation for 18-crown-6 molecules in the complex cation [Ln(fod)(2)(18-crown-6)](+) is 74 +/- 9 kJ mol(-1) at 363 K, which is a little more than the value of the free energy of activation 64 +/- 9 kJ mol(-1) at 363 K in the complex cation [Ln( ptfa)(2)(18-crown-6)](+).