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)n]3+ (Ln = La, Nd, Eu, Tb, or Yb; solv = H2O, MeOH, EtOH, iPrOH, or tBuOH; n = 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+.