Solid-State and Solution-State Coordination Chemistry of Lanthanide(III) Complexes with (Pyrazol-1-yl)acetic Acid

As a precursor of carboxyl-functionalized
task-specific ionic liquids (TSILs) for f-element separations, (pyrazol-1-yl)­acetic
acid (L) can be deprotonated as a functionalized pyrazolate
anion to coordinate with hard metal cations. However, the coordination
chemistry of L with f-elements remains unexplored. We
reacted L with lanthanides in aqueous solution at pH
= 5 and synthesized four lanthanide complexes of general formula [Ln­(L)3(H2O)2]·nH2O (1, Ln = La, n = 2; 2, Ln = Ce, n = 2; 3, Ln = Pr, n = 2; 4, Ln = Nd, n = 1).
All complexes were characterized by single crystal X-ray diffraction
analysis revealing one-dimensional chain formations. Two distinct
crystallographic structures are governed by the different coordination
modes of carboxylate groups in L: terminal bidentate
and bridging tridentate (1–3); terminal
bidentate, bridging bidentate, and tridentate coordination in 4. Comparison of the solid state UV–vis–NIR
diffuse reflectance spectra with solution state UV–vis–NIR
spectra suggests a different species in solution and solid state.
The different coordination in solid state and solution was verified
by distinctive 13C NMR signals of the carboxylate groups
in the solid state NMR.