Mechanistic study of solvent exchange reactions of manganese(II), iron(II), nickel(II), copper(II), and iron(III) ions in bulky 1,1,3,3-tetramethylurea (TMU) solvent by measurements of oxygen-17 NMR relaxation rates and chemical shifts: Complexation react

The solvent exchange between bulky TMU (1,1,3,3-tetramethylurea) molecules and the TMU molecules bound to metal cations (Mn(II), Fe(II), Ni(II), Cu(II), and Fe(III)) in neat TMU has been studied by the oxygen-17 NMR line-broadening and chemical shift method. The exchange rate constant (kex) at 25 °C, the activation enthalpy (ΔH‡), and the activation entropy (ΔS‡) for the solvent exchange reaction of the metal(II) ions and Fe(III) ion in TMU were obtained as follows: 4.3 × 106 s−1, 18.7 ± 2.2 kJ mol−1, −55 ± 7 J mol−1 K−1 for Mn(II) ion; 2.9 × 106 s−1, 31.1 ± 0.9 kJ mol−1, −16 ± 3 J mol−1 K−1 for Fe(II) ion; 8.5 × 105 s−1, 22.3 ± 1.8 kJ mol−1, −55 ± 3 J mol−1 K−1 for Ni(II); 5.6 × 106 s−1, 11.3 ± 1.4 kJ mol−1, −78 ± 4 J mol−1 K−1 for Cu(II); 6.6 × 105 s−1, 23.9 ± 1.2 kJ mol−1, −52 ± 4 J mol−1 K−1 for Fe(III), respectively. The trend in the activation parameters is as follows: the values of the activation enthalpy for the TMU exchange of a series of metal ions are lower than those for the DMF exchange of the corresponding metal ions and the activation entropy values are mostly negative. These findings indicate that the TMU solvent exchange proceeds via an associative-interchange mechanism.

Graphical abstractThe solvent exchange on the Mn(II), Fe(II), Ni(II), Cu(II), and Fe(III) ions in TMU (1,1,3,3-tetramethylurea) has been studied by the oxygen-17 NMR technique. The small activation enthalpies and negatively large activation entropies indicate that the TMU solvent exchange proceeds via an associative-interchange mechanism.Figure optionsDownload full-size imageDownload as PowerPoint slide