Synthesis, X-ray crystallography, spectroscopy, electrochemistry, thermal and kinetic study of uranyl Schiff base complexes

Abstract

Uranyl(VI) complexes [UO₂(L)(solvent)], where L denotes an asymmetric N₂O₂ Schiff base (salpyr, 3-MeOsalpyr, 4-MeOsalpyr, 5-MeOsalpyr, 5-Clsalpyr or 5-Brsalpyr; salpyr is N,N′-bis(salicyliden)-2,3-diaminopyridine), were synthesized and characterized in solution (UV–vis, ¹H NMR, cyclic voltammetry) and in the solid-state (X-ray crystallography, IR, TGA, C H N.). X-ray crystallography of UO₂(3-MeOsalpyr) revealed coordination of the uranyl by the tetradentate Schiff base and one disordered solvent, resulting in seven-coordinate uranium. Another disordered solvent was not coordinated. Cyclic voltammetry of [U^VIO₂(L)(solvent)] in acetonitrile was used to investigate the effect of the substituents of the Schiff base ligands on oxidation and reduction potential. The quasi-reversible redox reaction without any successive reactions was accelerated by groups with lesser electron withdrawing. We also investigated the kinetics and mechanism of the exchange reaction of the coordinated solvent with tributylphosphine using spectrophotometric method. The second-order rate constants at four temperatures and activation parameters showed an associative mechanism for all corresponding complexes with the following trend: UO₂(5-Clsalpyr) > UO₂(5-Brsalpyr) > UO₂(3-MeOsalpyr) > UO₂(4-MeOsalpyr) > UO₂(salpyr) > UO₂(5-MeOsalpyr). It was concluded that the steric and electronic properties of the complexes were important for the reaction rate.

Keywords:

• X-ray crystallography
• Uranyl Schiff base complex
• Kinetics of thermal decomposition
• Cyclic voltammetry
• Kinetics and mechanism

Acknowledgments

We are grateful to the Shiraz University Research Council for its financial support. The project P204/11/0809 of the Grant agency of the Czech Republic supported the crystallographic part of the work.