Strong and weak solvation steric effects on lanthanoid(III) ions in N,N-dimethylformamide-N,N-dimethylacetamide mixtures

Structural parameters of solvated neodymium(III), gadolinium(III), thulium(III) and ytterbium(III) ions in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and their mixtures have been obtained by an extended X-ray absorption fine structure (EXAFS) method. It revealed that the metal-oxygen bond length is shortened and the coordination number is thus decreased with increasing DMA content in the mixture. Interestingly, the decrease in the coordination number takes place evidently at a lower DMA content for the thulium(III) ion than the neodymium(III) ion, i.e., the geometry change is more favorable for the metal ion with a smaller ionic radius. Y-89 NMR chemical shifts also demonstrate that the coordination number decreases with increasing DMA content in the mixture, and the profile of varying coordination number is similar to that for the thulium(III) ion. This indicates that the strong solvation steric effect operates with both thulium(III) and yttrium(III) ions in the DMF-DMA mixture. Variations of metal-oxygen bond length for other metal systems in the mixture indicate that, relative to the thulium(III) ion, the gadolinium(III) ion experiences a less strong solvation steric effect, and the neodymium(III) ion even less. Structural parameters have also been determined for a series of lanthanoid(III) ions in both DMF and DMA. The Ln(III)-O bond length in DMA is generally shorter than that in DMF, and the difference is smallest for the lanthanum(III) ion, and largest for the lutetium(III) ion. The difference is thus larger for a metal ion with a smaller ionic radius, implying that the solvation steric hindrance is enhanced with decreasing ionic radius.