Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands

Abstract

Uranyl(vi) complexes with pentadentate N₃O₂-, N₂O₃- and N₂O₂S₁-donating Schiff base ligands, tBu,MeO-saldien-X^2- (X = NH, O and S), were synthesized and thoroughly characterized by ¹H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO₂(tBu,MeO-saldien-X) showed that the U-X bond strength follows U-O ≈ U-NH > U-S. Conditional stability constants (β _X) of UO₂(tBu,MeO-saldien-X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log β _X decrease in the order of UO₂(tBu,MeO-saldien-NH) (log β _NH = 10) > UO₂(tBu,MeO-saldien-O) (log β _O = 7.24) > UO₂(tBu,MeO-saldien-S) (log β _S = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO₂(tBu,MeO-saldien-X) suggested that the ionic character of U-X bonds decreases in the order of U-NH > U-O > U-S, while the covalency increases in the order U-O < U-NH < U-S. Redox potentials of all UO₂(tBu,MeO-saldien-X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U⁶⁺ of each UO₂(tBu,MeO-saldien-X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO₂(tBu,MeO-saldien-X) affects the coordination of tBu,MeO-saldien-X^2- with UO₂ ²⁺. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U-X interactions are more relevant to determine the bond strengths.

Fig. 1. Schematic structures of UO₂(R₁,R₂,–^Rsaldien) (a) and UO₂(tBu,MeO–saldien–X) complexes (UO₂(L_X)); (X = NH, O, S) (b).

Fig. 2. ORTEP views of UO₂(L_NH) (a), UO₂(L_O) (b), UO₂(L_S) (c). Ellipsoids are at 50% probability level. Hydrogen atoms and solvent molecules were omitted for clarify.

Fig. 3. UV-vis absorption spectra of the ethanol solutions of (a) L_NH²⁻, (b) L_O²⁻ and (c) L_S²⁻ at different total UO₂²⁺ concentrations under the presence of 0.4 mM NEt₃. Total concentration of L_X²⁻: 0.1 mM. Black and red lines are spectra of C_U/C_L = 0 and 1.0, respectively. Insets: plots of absorbance at 370 nm with an increase in C_U/C_L.

Fig. 4. Cyclic voltammograms for the redox couples of UO₂(L_X) (X = NH, O, S) in DMSO at 295 K. Concentration of the complex was adjusted to 1 mM and tetra-n-butylammonium perchlorate (0.1 M) was used as a supporting electrolyte. Scan rates are 100 mV s⁻¹.

Fig. 5. UV-vis-NIR spectral change of electrochemical reduction of UO₂(L_O) recorded at different applied potentials from −1.475 V to −1.665 V vs. Fc^0/+ (potential step: 0.015 V) in DMSO with 0.1 M TBAP at 295 K. Black and red bold curves represent absorption spectra of UO₂(L_O) and [UO₂(L_O)]⁻, respectively. Wavenumber regions: (a) 33 333–4500 cm⁻¹, (b) 20 000–4500 cm⁻¹. Inset: Nernstian plot calculated from absorbance at 24 630 cm⁻¹.

Fig. 6. UV-vis-NIR spectrum of UO₂(L_X) (black) and one-electron reduced complexes [UO₂(L_X)]⁻ (red) in DMSO containing 0.1 M tetra-n-butylammonium perchlorate at 295 K. [UO₂(L_NH)]^−/0 (a), [UO₂(L_O)]^−/0 (b) and [UO₂(L_S)]^−/0 (c). Inset: expended views of NIR region.

Fig. 7. Spin-density plots of [UO₂(L_NH)]⁻ (a), [UO₂(L_O)]⁻ (b), [UO₂(L_S)]⁻ (c). Spin density values of U atom are 1.09 for [UO₂(L_NH)]⁻, 1.09 for [UO₂(L_O)]⁻, 1.12 for [UO₂(L_S)]⁻.