Computational studies on the stability of nine-coordinate mixed ligand complexes formed by Zr(IV) and Hf(IV) with DOTA

Abstract

Ultra-pure form of Zr is used as a structural material for nuclear due to its lower absorption cross-section for thermal neutron absorption cross-section, while ultra-pure form of Hf is used in nuclear reaction controlling rod due to its higher absorption cross-section for thermal neutrons. However, obtaining ultra-pure forms of Zr and Hf is difficult due to their similar chemical properties and natural coexistence. Available separation techniques are inefficient, non-environmentally friendly and costly. This project aims to develop an environmentally-friendly, economical and efficient separation method with concepts used in mixed-ligand complex formation and fractional crystallization. Here, nine coordinate mixed ligand complexes formed by Zr(IV) and Hf(IV) with octadentate primary ligand, dodecane tetraacetate (DOTA) and with different monodentate secondary ligands (L) were studied with density functional theory (DFT) calculations. In literature, the crystal structure of [Zr(DOTA)] was recorded in which recrystallization was carried out in an aqueous medium. According to our calculations, Zr(IV) and Hf(IV) can form [M(DOTA)(H2O)] in aqueous medium. Hence, it can be assumed that [M(DOTA)(H2O)] forms during dissolution of [M(DOTA)] and H2O must be leaving during the crystal formation. If a mono-dentate ligand L, has a higher affinity than H2O, towards Zr(IV) or Hf(IV), and if the nine-coordinate mixed ligand complexes forming with DOTA by Zr(IV) and Hf(IV) show a significantly high stability difference, the most stable complex (formed by either Zr(IV) or Hf(IV)) with L and DOTA can be separated through selective precipitation or fractional crystallization. According to the calculations, pyridine (py), CN- and NH3 were identified as the secondary ligands which can form nine-coordinate mixed ligand complexes with DOTA, more stable than [M(DOTA)(H2O)]. Among them, py complexes show the highest stability. Therefore, further studies were continued on amino derivatives of pyridine, since the amine group increased the nucleophilicity of pyridine and it shows that complexes formed with 4-aminopyridine (4-aminopy) and DOTA show the highest stability and the highest difference in stability for Zr(IV) and Hf(IV) complexes. Since 4-aminopy shows higher affinity towards Zr(IV) than Hf(IV), in a medium containing DOTA and a limited amount of 4-aminopy, Zr(IV) is expected to form [Zr(DOTA)(4-aminopy)] while the Hf(IV) is expected to form [Hf(DOTA)(H2O)], which can be separated through fractional crystallization or selective precipitation.