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 conformationalinversion processes for the complexcations of Ce, Pr and Nd, respectively,were obtained. Quantitativeinvestigation 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)]+.