Mapping the Coordination Number and Coordination Geometry of Lanthanide Ions in Aqueous and Nonaqueous Solution Phases

Nonaqueous solvents are used for numerous lanthanide-containing processes, yet the solvation of lanthanides in general, and in nonaqueous solution specifically, is poorly understood. Lanthanide coordination geometries are driven by intraligand constraints and interligand repulsion. Studies on interligand restraints are largely limited to the lanthanide contraction and the "gadolinium break". In this work, we expand on interligand restraints in a series of lanthanides, from large (La³⁺) to small (Yb³⁺), and solvents from small (H₂O) to bulky (tBuOH) by means of X-ray total scattering, density functional theory (DFT), optical spectroscopy, and multiconfigurational computational methods. We find from X-ray total scattering that the average Ln-O bond distance in [Ln(solv)_<i>n</i>]³⁺ (Ln = La, Nd, Eu, Tb, or Yb; solv = H₂O, MeOH, EtOH, iPrOH, or tBuOH; <i>n</i> = 7, 8, or 9) decreases as the lanthanide ionic radius decreases but remains constant as the solvent bulk is increased. This is rationalized via DFT by a decrease in the coordination number counteracting the inherent increase in the Ln-O bond distance as solvent bulk is increased, resulting in no changes in the average Ln-O bond distance being observed in experiments. These results are confirmed by qualitative shifts in the spectral shape from absorption and luminescence spectroscopy and by direct comparison with simulated optical spectra for Yb³⁺.