Acid Mediated In-Situ Catalytic Cleavage/Making of C=N Bonds in Bis-Hydrazone Uranyl Complexes: Exploration of Photophysical, Electrochemical, Dye Sorption Properties and DFT Studies

Two new uranium complexes [UO₂L²(H₂O)] (where L²=diacetyl bis-isonicotinoyl hydrazone) complex 1, and [UO₂(L¹)₂] (where L¹=diacetylmonoxime isonicotinoylhydrazone) complex 2, have been synthesized by precisely adjusting the pH values of the reaction system. Complex 1 was formed in an acidic ethanolic solution (pH=3) through an in-situ acid catalytic reaction of diacetylmonoxime isonicotinoylhydrazone and UO₂(NO₃)₂.6H₂O, while complex 2 was acquired in a neutral medium (pH=7) through self-assembly of two ligands and one uranium (VI) metal centre. Both complexes were thoroughly characterized by elemental analysis, UV-Visible spectroscopy, IR spectroscopy, NMR, TGA, cyclic voltammetry, and powder X-ray diffraction. The studies indicate that the bis-hydrazone in complex 1 is a dianionic tetradentate N₂O₂ donor, whereas in complex 2 it is a monoanionic tridendate N₂O donor. The formation of complex 1 was further confirmed by a single-crystal X-ray diffraction study, which revealed the role of a water molecule in forming a pentagonal bipyramidal geometry of [UO₂L²(H₂O)]. Photophysical studies revealed that complexes 1 and 2 exhibited low band gap values of 2.07 and 2.12 eV, respectively, indicating their semiconducting nature. From the CV measurements, the higher negative potentials of the complexes 1 & 2 {1, −0.6 V and 2, −0.5 V vs. ferrocenium/ferrocene (Fc⁺/Fc)} in DMSO are assigned to the presence of {UO₂}²⁺/{UO₂}⁺ couple signifying their redox active nature. Dye adsorption studies show that both the complexes were able to remove 90 % of Methylene blue (MB) and Rhodamine B (RhB) from the solutions with the concentration of 10⁻⁵ M. The structural insights and electronic properties of Ligand HL¹ and complex 1 were further explored by density functional theory (DFT) calculations.