A crystallographic investigation of multidentate ligand hafnium(IV) halido complexes
Loading...
Date
2021-11
Authors
Viljoen, Johannes Augustinus
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Hafnium, the chemical element with atomic number 72 in the titanium-triad on the periodic table, was first discovered by Dirk Coster and George von Hevesy in 1923 by analysing X-Ray spectra of Norwegian zircon samples (sea sand). Hafnium is considered relatively abundant and comprises about three parts per million by weight of the earth's crust. Unfortunately, hafnium only occurs naturally in zirconium ores at an approximate ratio of 1:50 and has almost identical general chemical properties to that of zirconium. Zirconium has a very desirable application in the nuclear industry as a cladding material for nuclear fuel rods due to its low affinity for thermal neutrons. However, for zirconium to be used as a cladding material in nuclear reactors, it must be virtually hafnium free since hafnium has a 600 times larger affinity for thermal neutrons than that of zirconium, turning it into a “poison” in zirconium cladding materials. However, this high affinity for thermal neutrons is exactly why pure Hf makes an excellent shielding material. Therefore, for hafnium and zirconium to be utilised in the nuclear industry, it is apparent why the effective and economically viable separation of these metals to their pure chemical state is vital. The principle aim of this study is focused on studying the solid-state and solution behaviour of hafnium(IV), with various N- and O-donating multidentate ligand systems, in order to obtain valuable new data that could potentially be used to compare to zirconium(IV) to evaluate if metal separation is feasible for industrial applications. Therefore, in this study, the solid-state and solution behaviour of novel hafnium(IV) and zirconium(IV) coordination compounds, obtained from a systematic range of N- and O- bi- and multidentate ligands, is investigated. A detailed description of the synthesis of the hafnium(IV) and zirconium(IV) complexes containing different N,O- and O,O’-bidentate ligand systems as well as N,O,O,N- and N,N’,N’,N-tetradentate ligands are reported and characterised by means of IR, UV/Vis and NMR (1H and 13C) spectroscopies. Moreover, it is clearly illustrated by this study that it is possible to synthesise hafnium(IV) and zirconium(IV) complexes under simple bench-top aerobic conditions. Furthermore, the solid-state structural characterisation by means of single crystal X-Ray Diffraction studies of ten of these synthesised complexes is described in detail. Thus, six novel hafnium(IV)-oxinato complexes containing a systematic range of substituents on the outer periphery of the ligand are firstly presented: (1) [Hf(Ox)4]·2(C7H8), (2) [Hf(5-ClOx)4]·2(C7H8), (3) [Hf(diMeOx)4]·2(DMF), (4) [Hf(diClOx)4]·3DMF, (5) [Hf(diBrOx)4]·3DMF and (6) [Hf(diIOx)4]·DMF and described. The complexes contain non- and substituted 8-hydroxyquinoline (OxH) ligands [with (5-ClOxH) = 5-chloro-8-hydroxyquinoline, (diMeOxH) = 5,7-dimethyl-8-hydroxy-quinoline, (diClOxH) = 5,7-dichloro-8-hydroxyquinoline, (diBrOxH = 5,7-dibromo-8-hydroxyquinoline, (diIOxH) = 5,7-diiodo-8-hydroxyquinoline], and are discussed and compared with regard to the intimate geometric environment around the hafnium(IV) metal centre. It is clearly illustrated by this study that tetrakis-(oxinato)hafnium(IV) complexes prefer the square-antiprismatic coordination geometry in the D2-corner clipped isomer form. It is further shown that all the hafnium(IV) -and zirconium(IV) oxine complexes are very similar with relation to bond lengths, angles and packing modes. Secondly, two new hafnium(IV)-β-diketonato complexes – (7) [Hf(dbm)4] and (8) [Hf(OH)(tfba)3]2·2DMF – are structurally evaluated and described [where (dbmH) = dibenzoylmethane and (tfbaH) = 4,4,4-trifluoro-1-phenyl-1,3-butanedione]. Focus is placed on the influence of ligands when increasing the metallocycle from five (oxine-type ligands) to six (acac-type ligands) and using acac-type ligands containing different electron-donating properties. The effect of electron-withdrawing substituents on the ligand backbone with regards to coordination mode and coordination geometry. These examples show and conclude that hafnium-β-diketonato complexes also prefer a square-antiprismatic coordination geometry in the D2-corner clipped isomer form. Moreover, all the hafnium(IV)- and zirconium(IV)-β-diketonato complexes are very similar with respect to their general geometry and corresponding bond lengths, angles and packing modes. Similar to the oxine-type complexes, this suggests that hafnium(IV) and zirconium(IV) has a certain preference for the chelation sites of the coordinating atoms, regardless of the steric or electronic properties of the ligands as a whole. Thirdly, a novel hafnium(IV) and zirconium(III) complex containing a N,O,O,N- and N,N’,N’,N-tetradentate ligand, respectively – (9) [Hf(salophen)2·2(C7H8), (10) [Zr(pbp)(H2O)2]2NO3·(MeOH) – are structurally evaluated and compared with regard to the intimate geometric environment around the metal centres [where (salophen) =N,N'-bis(salicylidene)-1,2-phenylene-diamine & (pbpH) = N,N'-(1,2-phenylene)bis-(pyridine-2-carboxamide)]. It was found that the eight coordinated hafnium(IV) complex favours a square-antiprismatic coordination polyhedron. However, the six coordinated zirconium(III) complex formed a distorted octahedron as coordination polyhedron around the metal centre, as expected for many six coordinated metal complexes. Finally, preliminary liquid-liquid extraction experiments are described herein, and it is concluded to in principle be considered a preliminary proof-of-concept for separation. The preliminary extraction investigation reported includes liquid-liquid extractions of zirconium- and hafnium-oxine complexes from methanol into dichloromethane and competition extractions, as qualitatively evaluated by (I) UV-Vis spectroscopy and (II) inductively coupled plasma optical emission spectroscopy.
Description
Keywords
Thesis (Ph.D. (Chemistry))--University of the Free State, 2021, Hafnium, Titanium-triad, Zirconium ores, Chemistry