Solid-State and Solution-State Coordination Chemistry of Lanthanide(III) Complexes with α‑Hydroxyisobutyric Acid

Despite the wide range of applications of α-hydroxyisobutyric
acid (HIBA) in biochemical processes, pharmaceutical formulations,
and group and elemental separations of lanthanides and actinides,
the structures and geometries of lanthanide–HIBA complexes
are still not well understood. We reacted HIBA with lanthanides in
aqueous solution at pH = 5 and synthesized 14 lanthanide–HIBA
complexes of the formula [Ln­(HIBA)2(H2O)2]­(NO3)·H2O (Ln = La (1), Ce (2), Pr (3), Nd (4),
Sm (5), Eu (6), Gd (7), Tb
(8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14)), isolating single crystals (1–7, 10, and 11) and powders (8, 9, and 12–14). Both single-crystal
and powder X-ray diffraction studies reveal a two-dimensional extended
structure across the entire lanthanide series. The environment around
the eight-coordinated Ln­(III) atom is best described as a distorted
dodecahedron, where HIBA acts as a monoanionic tridentate ligand with
one carboxylato oxygen atom and one hydroxyl oxygen atom chelating
to one Ln­(III) center. The carboxylato oxygen atom from a second HIBA
ligand bridges to a neighboring Ln­(III) atom to form a two-dimensional
extended structure. While the coordination mode for HIBA is identical
across the lanthanide series, three different structure types are
found for La, Ce–Ho, and Er–Lu. Solution characterization
using 13C NMR further confirmed a single solution complex
under the crystallization conditions. Raman and UV–vis–NIR
absorbance and diffuse reflectance spectra of HIBA–Ln­(III)
complexes were also measured.