Structural and Thermodynamic Study of the Complexes
of Nd(III) with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetramethyl-3-oxa-glutaramide
and the Acid Analogues
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Abstract
The thermodynamics of Nd(III) complexes
with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethyl-3-oxa-glutaramide (TMOGA, L<sup>I</sup>), <i>N</i>,<i>N</i>-dimethyl-3-oxa-glutaramic
acid (DMOGA,
HL<sup>II</sup>), and oxydiacetic acid (ODA, H<sub>2</sub>L<sup>III</sup>) 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)/L<sup>III</sup> > Nd(III)/L<sup>II</sup> > Nd(III)/L<sup>I</sup>. 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 L<sup>III</sup> to L<sup>II</sup>,
and to L<sup>I</sup>, 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(L<sup>I</sup>)<sub>3</sub>(ClO<sub>4</sub>)<sub>3</sub> (<b>I</b>), Nd(L<sup>I</sup>)<sub>3</sub>(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub> (<b>II</b>), and Nd(L<sup>II</sup>)<sub>3</sub>(H<sub>2</sub>O)<sub>7.5</sub> (<b>III</b>), were determined by single-crystal X-ray diffraction
and compared with the structure of a 1:3 Nd(III)/L<sup>III</sup> complex
in the literature, Na<sub>3</sub>NdL<sup>III</sup><sub>3</sub>(NaClO<sub>4</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub> (<b>I</b><b>V</b>). 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 L<sup>I</sup>, L<sup>II</sup>, and L<sup>III</sup>, respectively, implying that the Nd(III) complexes with the three
ligands have similar coordination geometries in aqueous solutions,
as observed in the solids