Structural and Thermodynamic Study of the Complexes of Nd(III) with N,N,N′,N′‑Tetramethyl-3-oxa-glutaramide and the Acid Analogues

The thermodynamics of Nd­(III) complexes with N,N,N′,N′-tetramethyl-3-oxa-glutaramide (TMOGA, LI), N,N-dimethyl-3-oxa-glutaramic acid (DMOGA, HLII), and oxydiacetic acid (ODA, H2LIII) in aqueous solutions was studied. Stability constants, enthalpies, and entropies of complexation were determined by spectrophotometry, potentiometry, and calorimetry. The stability constants of corresponding Nd­(III) complexes decrease in the following order: Nd­(III)/LIII > Nd­(III)/LII > Nd­(III)/LI. For all complexes, the enthalpies of complexation are negative and the entropies of complexation are positive, indicating that the complexation is driven by both enthalpy and entropy. Furthermore, from LIII to LII, and to LI, the enthalpy of complexation becomes more exothermic and the entropy of complexation less positive, suggesting that the substitution of a carboxylate group with an amide group on the ligands enhances the enthalpy-driven force but weakens the entropy-driven force of the complexation with Nd­(III). Crystal structures of three 1:3 Nd­(III) complexes, Nd­(LI)3(ClO4)3 (I), Nd­(LI)3(NO3)3(H2O)2 (II), and Nd­(LII)3(H2O)7.5 (III), were determined by single-crystal X-ray diffraction and compared with the structure of a 1:3 Nd­(III)/LIII complex in the literature, Na3NdLIII3(NaClO4)2(H2O)6 (IV). In all four structures, the ligands are tridentate and Nd­(III) is nine-coordinated with similar distorted tricapped trigonal prism geometry by three ether oxygen atoms capped on the three faces of the prism, and six oxygen atoms from the ketone group or carboxyl group at the corners. The absorption spectra of Nd­(III) in solutions showed very similar patterns as Nd­(III) formed successive 1:1, 1:2, and 1:3 complexes with LI, LII, and LIII, respectively, implying that the Nd­(III) complexes with the three ligands have similar coordination geometries in aqueous solutions, as observed in the solids.