Effects of Substituents on the Molecular Structure and Redox Behavior of Uranyl(V/VI) Complexes with N₃O₂-Donating Schiff Base Ligands

Abstract

Uranyl(VI) complexes with pentadentate N 3 O 2 -donating Schiff base ligands having various substituents at the ortho (R 1 ) and/or para (R 2 ) positions on phenolate moieties, R 1 ,R 2 -Me saldien 2− , were synthesized and thoroughly characterized by 1 H nuclear magnetic resonance, infrared, elemental analysis, and single-crystal X-ray diffraction. Molecular structures of UO 2 (R 1 ,R 2 -Me saldien) are more or less affected by the electron-donating or -withdrawing nature of the substituents. The redox behavior of all UO 2 (R 1 ,R 2 -Me saldien) complexes was investigated to understand how substituents introduced onto the ligand affect the redox behavior of these uranyl(VI) complexes. As a result, the redox potentials of UO 2 (R 1 ,R 2 -Me saldien) in dimethyl sulfoxide increased from −1.590 to −1.213 V with an increase in the electron-withdrawing nature of the substituents at the R 1 and R 2 positions. The spectroelectrochemical measurements and theoretical calculation [density functional theory (DFT) and timedependent DFT calculations] revealed that the center U 6+ of each UO 2 (R 1 ,R 2 -Me saldien) complex undergoes one-electron reduction to afford the corresponding uranyl(V) complex, [UO 2 (R 1 ,R 2 -Me saldien)] − , regardless of the difference in the substituents. Consequently, the redox active center of uranyl(VI) complexes seems not to be governed by the redox potentials but to be determined by whether the LUMO is centered on a U 5f orbital or on one π* orbital of a surrounding ligand.

Synthesis and characterization of a uranyl(vi) complex with 2,6-pyridine-bis(methylaminophenolato) and its ligand-centred aerobic oxidation mechanism to a diimino derivative

“…25 The most diffuse basis functions on uranium with the exponent 0.005 (all s, p, d, and f type functions) were omitted as in previous studies. [26][27][28][29] The 6-311G(d) basis sets were used for other elements (H, C, N, and O).…”

“…After this reaction, 1 complexes with Schiff base ligands (9.4-10.5 ppm). 29,32 The peak integral of this signal corresponds to two H atoms in the new species. The most reliable The same reaction was also observed in pyridine.…”

“…As summarized in Table S2 (ESI †), the UuO ax (U-O(1), U-O(2)) bond lengths of 2 are typical of UO 2 2+ complexes reported elsewhere. 29,30,32 The ( phenolate) ligands. 29,30 The C(7)-N(1) and C(13)-N(3) bond lengths of 2 well agree with that of a typical CvN double bond.…”

“…29,30,32 The ( phenolate) ligands. 29,30 The C(7)-N(1) and C(13)-N(3) bond lengths of 2 well agree with that of a typical CvN double bond. 29,32 Furthermore, the mean bond angles around N(1), N(3), C(7), and C( 13) are close to 120°, indicating the sp 2 character of these atoms involved in the CvN moieties of 2.…”

“…S27 †). [25][26][27][28][29] Herein, the tert-butyl groups were replaced with hydrogens to reduce the computational cost. An O 2 molecule as a reactant was placed in the vicinity of the optimized structure of 1 ( 3 S1, Fig.…”