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

초록

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. Two uranium complexes have been synthesized using diacetylmonoxime isonicotinoylhydrazone as a ligand. In this study, the formation of two different complexes has been observed under two different pH conditions. At neutral pH, a M(L12) type structure is formed by the association of two ligands with the metal. Whereas, in acidic medium (pH=3), the same ligand is found to be involved in the acid catalytic hydrolysis of C=N and formation of new C=N bond to result in a new type of ML2 complex. Photophysical, electrochemical, and dye sorption properties have been successfully explored. Structural optimization of the ligand and complex was carried out by density functional theory (DFT) calculations. image

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