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.