Solvation structure and bromo complexation of neodymium(III) and yttrium(III) ions in solvent mixtures of N,N-dimethylformamide and N,N-dimethylacetamide

Bromo complexation of neodymium(III) and yttrium(III) in solvent mixtures of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) has been studied by precise titration calorimetry, 89Y NMR and extended X-ray absorption fine structure (EXAFS). With regard to neodymium(III), calorimetric data in the mixture of DMA content x are explained satisfactorily in terms of formation of mononuclear NdBrn(3-n)+(n=1, 2 at x=0.5 and n=1, 2, 3 at x=0.6, 0.75, 0.85). With yttrium(III), calorimetric data are also explained well in terms of formation of mononuclear YBrn(3-n)+(n=1, 2 at x=0.25, 0.5, 0.6 and n=1, 2, 3 at x=0.65, 0.75, 0.85). Formation enthalpy and entropy values of the monobromo complex remain practically unchanged up to x≈0.5, and then significantly increase with increasing x. It is thus suggested in both metal(III) systems that outer-sphere bromo complexes are formed in the DMF-rich mixture, while inner-sphere bromo complexes are formed in the DMA-rich mixture in equilibrium with the outer-sphere ones, and the formation of inner-sphere complexes becomes more favorable with increasing DMA content. The marked difference in the complexation in DMF and DMA may be ascribed to the solvation steric effect. The solvation steric effect is enhanced with increasing DMA content leading to an enhanced complexation and a simultaneous geometry transition from outer- to inner-sphere coordination of the bromide ion. Evidence of the geometry transition has been obtained by EXAFS for neodymium(III), and by 89Y NMR for yttrium(III). Interestingly, although ionic radii of neodymium(III) and yttrium(III) ions are significantly different, no appreciable metal-ion dependence is found with the geometry transition from outer- to inner-sphere bromo complex. This indicates that the geometry transition of the metal(III) solvate ion from eight- to seven-coordination is not always the necessary condition for the transition from outer- to inner-sphere complexation.