Two Similar Uranyl Complexes with a “Salen-Type” Schiff Base as Ligand and Different Coordinated Solvents: Synthetic, Structural, Spectroscopic and Physical Properties

The coordination chemistry of “salen-type” Schiff bases with the uranyl ion, trans-{UVIO2}2+, attracts the interest of several inorganic chemistry groups worldwide for a variety of reasons. The full synthetic investigation of the {UO2}2+/H2L reaction system, where H2L is bis(2-hydroxyacetophenone)ethylenediamine, has provided access to complexes [UO2(L)(EtOH)] (1) and [UO2(L)(DMF)] (2) in moderate to good yields. The molecular structures of the two complexes are similar. The UVI atoms are bonded to five oxygen and two nitrogen atoms in a distorted pentagonal bipyramidal geometry. The two uranyl oxo(or oxido) atoms occupy the axial positions, and the {O=U=O}2+ moiety is almost linear. The equatorial donor atoms are the two oxygens and the two nitrogens from the tetradentate chelating (1.1111 using Harris notation) L2− ligand, and the oxygen atom of the coordinated solvent molecule. H-bonded dimers of 1 exist in its crystal structure. The complexes were fully studied in the solid state by IR, Raman, UV/Vis (diffuse reflectance) and emission spectroscopies, and the data are discussed in terms of the known structural data of the complexes and the coordination modes of the ligands. The structures of the complexes persist in solution as evidenced by NMR (1H, 13C{1H}) and UV/Vis spectroscopies, as well as by molar conductivity data. Complexes 1 and 2 exhibit moderate photocatalytic activity towards the degradation of the model organic dye methylene blue under continuous UV irradiation in aqueous media. The reaction kinetics were fitted using the Langmuir-Hinshelwood pseudo-first-order model. Combined IR and powder X-ray diffraction data show that the photocatalyst 1 remains unchanged after the photocatalytic experiment, whereas 2 undergoes DMF leaching. Based on literature reports, a simplified single-electron transfer mechanism has been proposed for the photocatalytic activity.