A solid state and mechanistic study of multidentate ligand zirconium(IV) halido complexes

[摘要] English: Zirconium and hafnium, the chemical twins in the titanium triad of the periodic table, are chemically very similar, but exhibit a significant difference in their nuclear properties, and also most noteworthy, their application in industry. Zirconium, with its very low affinity for thermal neutrons (radioactive energy), high thermal stability and exceptional anti-corrosive properties, is widely used as cladding material for nuclear reactor fuel rods. Hafnium, on the other hand, with its very high affinity for thermal neutrons is most often employed as control rods, used for regulating the rate of fission in nuclear reactors. For this application alone, it is apparent why the separation of these metals to their chemically pure state, is so important. Even the smallest impurity of one metal in the other would seriously degrade the ability of the metal to function in its particular role in a nuclear reactor. In this study, the solid state and solution behaviour of zirconium(IV) coordination compounds, containing N- and O-donating multidentate ligands, is investigated. A detailed description of the synthesis of 25 zirconium(IV) complexes with three ligand families �?O,O'-donating, oxines and pyridines �?are reported and characterised by means of IR, UV/Vis and NMR (1H & 13C) spectroscopies. Furthermore, the solid state structural characterisation, by means of single crystal X-Ray Diffraction spectroscopy, of eight of these synthesised complexes is described in detail. Six novel oxine-type complexes �?[Zr(diClOx)4]�?DMF, [Zr(5-ClOx)4]�?DMF, [ZrCl(CliOx)2(DMF)2O]2∙DMF, [Zr(diMeOx)4] �?DMF, [Zr(5-NO2Ox)4] and [Zr(Pic)4]�?H2O �?are discussed and compared with regard to the intimate geometric environment around the zirconium(IV) metal centre [where 5,7-Dichloro-8-hydroxyquinoline (diClOxH), 5-Chloro-8-hydroxyquinoline (5-ClOxH), 5-Chloro-7-iodo-8-hydroxyquinoline (CliOxH), 5,7-Dimethyl-8-hydroxyquinoline (diMeOxH), 5-Nitro-8-hydroxyquinoline (5-NO2OxH), 2-Picolinic acid (PicH)]. It is shown here that these structures also exhibit the classic square antiprismatic coordination polyhedra as expected for the O,O'-donating acetylacetone-type complexes of zirconium(IV). The redetermination for the structures [Zr(Trop)4]∙DMF and [Zr(DBM)4] is also included and further discussed in detail [Tropolone (TropH), Dibenzoylmethane (DBM)]. In these examples, it is shown and concluded that zirconium(IV) complexes exhibit a natural tendency to form tetrakis- or eight-coordinate crystalline products, as preference, with a square antiprismatic chelation environment around the metal centre in most of the cases. From these findings it was postulated that, for this metal centre, the susceptibility of chelation geometry to intermolecular forces seems greater than that of ligand internal geometry and is primarily governed by the zirconium(IV) centre itself. In addition, a solution kinetic study (utilizing UV/Vis Absorbance spectroscopy) on the formation mechanism of three oxine complexes synthesised and crystallographically characterised, is also included. The rudimentary findings for [Zr(diClOx)4], [Zr(5-ClOx)4] and [Zr(diMeOx)4] (from ZrCl4 as reactant) is shown to correspond well with that of the [Zr(ox)4] complex evaluated in the preliminary M.Sc. study. The entering ligands were selected to span a range of increasing donor ability utilising both chloro and methyl substituents on the oxine backbone. In all cases the same general intricate process of bidentate ligand coordination was observed. This process involves a fast initial reaction, followed by three slower multi-phase reaction steps wherein it is assumed that the chloride ligand is liberated as HCl (H+ from oxH) in the ring-closing step in each of the four processes. In total, the reactions span about a five order-of-magnitude range, indicating a huge change in affecting all four the chlorido ligands to be substituted to form the corresponding tetrakis-oxine complexes. Where the first step showed an increase by more than 30 times as the donor ability of the entering ligand increased, the subsequent following three reactions indicated approximately one order-of-magnitude decrease in ligation rate. Equilibrium suppression studies were also performed by means of stepwise increasing the concentration of the leaving ligand, in this case Cl-, by the addition of a spectator organic salt, PPh4+Cl-. The findings in this case show that all four reaction steps for all three zirconium(IV) complexes are significantly affected and restrained by this equilibrium manipulation process. Next, theoretical optimization, by means of computational chemistry techniques, of two different families of zirconium(IV) complexes was performed and the subsequent results evaluated by comparison with the structural data obtained from single crystal studies. The two families in question, the tetrakis(acetylacetonato-κ2O,O')zirconium(IV)- and tetrakis(oxine-κ2N,O)zirconium(IV) type examples, were each selected to represent the O,O- and N,O- coordination environments of the metal polyhedron. Fair correlation is observed for the [Zr(Acac)4] type complexes, that is [Zr(acac)4], [Zr(hFacac)4] & [Zr(DBM)4], with their optimized counterparts. However, for [Zr(acac)4] a different coordination isomer was obtained from the optimization. This observation can be attributed to the fact that theoretical optimization of molecular structures assumes a gas phase environment, without inter- or intramolecular forces governing lattice stabilization/packing. It can therefore be postulated with more certainty that zirconium(IV)complex structures experience a more significant influence from these interactions than previously considered. Finally, the optimization of three zirconium(IV) oxine complexes�?[Zr(ox)4], [Zr(diMeOx)4] & [Zr(diClOx)4] �?was also accomplished to elucidate whether or not these ligands, with a more rigid frame and packing placement, would show similar optimisation. This was consequently illustrated to an acceptable accuracy with good correlation between crystallographically characterised structures and their theoretically optimised counterparts.

[发布机构] University of the Free State