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 (<b>L</b>) can be deprotonated as a functionalized pyrazolate anion to coordinate with hard metal cations. However, the coordination chemistry of <b>L</b> with f-elements remains unexplored. We reacted <b>L</b> with lanthanides in aqueous solution at pH = 5 and synthesized four lanthanide complexes of general formula [Ln­(L)₃(H₂O)₂]·<i>n</i>H₂O (<b>1</b>, Ln = La, <i>n</i> = 2; <b>2</b>, Ln = Ce, <i>n</i> = 2; <b>3</b>, Ln = Pr, <i>n</i> = 2; <b>4</b>, Ln = Nd, <i>n</i> = 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 <b>L</b>: terminal bidentate and bridging tridentate (<b>1</b>–<b>3</b>); terminal bidentate, bridging bidentate, and tridentate coordination in <b>4</b>. 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.