High oxidation state niobium and tantalum coordination chemistry: a solution and solid state investigation

English: Niobium and tantalum, chemical twins of the vanadium triad of the periodic table, are notoriously difficult to separate from one another and from their naturally occurring ores due to their near identical chemical properties. The significance of the separation of these two elements lies in the value and application of these elements in various fields of uses, especially noting the nuclear industry. Niobium with its high melting point, strength, resistance properties to chemical attack and the low neutron absorption cross-section (NAC) is ideally suited when alloyed with zirconium for cladding material in control rods of nuclear reactors to prevent leakage of nuclear reactive materials. Tantalum, on the other hand, has a much more limited application in the nuclear industry. It is mainly used in combination with carbon to form tantalum carbide which is used as a lining agent within the nuclear reactor (due to the corrosion resistance of tantalum). Due to that fact tantalum and niobium are always found together in mineral ores, tantalum is actually seen as a “pollutant” by nuclear chemists. This is why an efficient separation method is required because 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. The principle aim of this study was to gain insight into the coordination and kinetic behaviour of Nb(V)- and Ta(V)-tropolonato and -acetylacetonato complexes. A detailed description of the synthesis of ten niobium(V)- and eight tantalum(V) complexes with the two ligand families (O,O’-donating) are reported and characterized by means of IR, UV/Vis and NMR (1H, 13C, 19F) spectroscopies. Furthermore, the solid state structural characterization, by means of single crystal XRay diffraction spectroscopy, yielded eleven of these synthesized compounds which were described in detail in three separate sections. The original focus of this crystallographic investigation was placed on the characterization of the more robust Nb(V)- and Ta(V) synthons, (NEt4)[NbCl6] and (NEt4)[TaCl6] and the factors that govern their stability for coordinative purposes. Secondly, six solid-state crystal structures of Nb(V)-β-diketonates (NEt4)[NbOCl3(ttfa)], (NEt4)[NbOCl3(tffa)], (NEt4)[NbOCl3(ntfa)], (NEt4)[NbOCl3(btfa)], (NEt4)[NbOCl3(hffa)] and (NEt4)[NbOCl3(3Cl-acac)] (where ttfa = thenoyl trifluoroacetylacetonato, tffa = trifluorofurylacetylacetonato, ntfa = naphtyltrifluoro acetylacetonato, btfa = benzoyltrifluoroacetonato, hfaa = hexafluoroacetylacetonato and 3Cl-acac = 3-chloroacetylacetonato) were discussed and compared with regard to the intimate geometric environment around the niobium(V) metal centre. And finally, the in-depth characterization relating to the solid state tris- and tetrakiscoordination modes of three M(V)-tropolonato (Trop) complexes, [NbO(Trop)3], [Ta(Trop)4Cl] and [Nb(Trop)4Cl] was effective in relating solid-state coordination preferences with complex stability in solution. To further evaluate the electronic environment experienced by the niobium(V) and tantalum(V) centres in these complexes, a kinetic study of the substitution reactions of [NbCl6]- and [TaCl6]- with a range of β-diketones as entering ligand was undertaken. The data reported from this study could be used in a systematic way to successfully derive an overall reaction mechanism and rate law, which accounts for all current experimental observations. It was also found that for a specific ligand, at fixed [Cl-]free and [H2O]free values, the first term in of the complicated rate law could actually be simplified to a constant value, defined by kfwd. From the obtained results and observations noted during this study, by using this equation, various previously unknown chemical characteristics for these systems such as enhancement of reaction rates by hydrolization could be quantified. Additionally, a relationship could also be established between the pKa of the uncoordinated β-diketonato entering ligands and the rate of ligand substitution. These substitution rates were seen to increase significantly as the pKa of the free ligand was decreased. This was ascribed to a decrease in electron density at the metal centre, which causes it to be more prone to nucleophilic attack. It was seen that the four order-of-magnitude increase in Bronsted basicity of the free bidentate ligands results in an eight and six times decrease in substitution reactivity at the Nb(V) and Ta(V) centres respectively. This investigation concluded by comparing different properties associated with the niobium(V)- and tantalum(V) complexes and relating those properties to the changes that were introduced in each complex by using different coordinating β-diketonato ligands. Several trends were noted as a result of the differing electronic properties of the various β-diketonate ligands. It was noted that as more electron-donating substituents (higher pKa) are encountered on the acac backbone of the complex causes a substantial influence on the oxido, trans effect of the compounds, as well as seemingly causing an increase in intermolecular hydrogen bonding interactions. In contrast, as electron-withdrawing capabilities of the substituents on the β-diketonate ligand are increased more electron density will be encountered within the periphery of this ligand. This in turn decreases the amount of intermolecular hydrogen bonding interactions, which seems to impact the sublimation properties of the compounds. If all of these fragments of information are combined it was found that by increasing electron-donation capabilities of the β-diketonate substituent, improved recovery of niobium can be expected from sublimation separation studies. All of these observations give a better insight into the properties that govern the chemical and physical properties of these systems. These deductions that have been discussed, are only a proof of concept regarding the significance of a study to elucidate the intimate geometric nature and characteristics of Nb(V)- and Ta(V) coordination with organic ligands for separation purposes.