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

Abstract

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.

Afrikaans: Niobium en tantaal, chemiese tweeling van die vanadium drietal van die periodieke tabel, is berug daarvoor dat skeiding tussen die twee en vanuit hulle natuurlike ertse besonder moeilik is danksy hulle amper identiese chemiese eienskappe. Die belangrikheid van die skeiding van hierdie twee elemente lê in die waarde en toepassing van hierdie elemente in verskillende velde van gebruik, veral in die kern bedryf. Niobium, met sy hoë smeltpunt, sterkte, weerstandseienskappe teen chemiese aanval en die lae neutron absorpsie deursnee (NAD) is ideaal vir gebruik as bekleding materiaal in beheerstawe van kernreaktore om lekkasie van kernreaktiewe materiaal te voorkom wanneer dit met sirkonium gelegeer is. Daarenteen het tantaal ʼn baie meer beperkte toepassing in die kernnywerheid. Dit word grotendeels in kombinasie met koolstof gebruik om tantaalkarbied te vorm wat as voeringsagent binne die kernreaktor gebruik word weens die roesweerstand van tantaal. Aangesien tantaal en niobium altyd saam in mineraalerts gevind word, word tantaal as “besoedeling” beskou deur kernchemici. ʼn Effektiewe skeidingsmetode word dus vereis aangesien selfs die kleinste onsuiwerheid van een metaal in die ander die vermoë van die metaal om sy funksie in die kernreaktor te verrig ernstig sal belemmer. Die hoofdoel van hierdie studie was om insig in te win rakende die koördinering en kinetiese gedrag van Nb(V)- en Ta(V)-tropolonato en -asetielasetonato komplekse. ʼn Breedvoerige beskrywing van die sintese van tien niobium(V) en agt tantaal(V) komplekse met die twee ligandfamilies (O,O’-skenkend) word gerapporteer; die komplekse is gekarakteriseer deur middel van IR, UV/Vis en KMR (1H, 13C, 19F) spektroskopie. Daarbenewens het die vaste toestand struktuurkarakterisering deur middel van enkelkristal X-straal diffraksie elf van hierdie vervaardigde verbindings opgelewer en dit word breedvoerig in drie afdelings bespreek. Die oorspronklike fokus van hierdie kristallografiese ondersoek was die karakterisering van die meer robuuste Nb(V)- en Ta(V) sintone, (NEt4)[NbCl6] en (NEt4)[TaCl6], en die faktore wat hulle stabiliteit vir koördineringsdoeleindes beheer. Tweedens is ses vaste toestand kristalstrukture Nb(V)-β-diketonate - (NEt4)[NbOCl3(ttfa)], (NEt4)[NbOCl3(tffa)], (NEt4)[NbOCl3(ntfa)], (NEt4)[NbOCl3(btfa)], (NEt4)[NbOCl3(hffa)] en (NEt4)[NbOCl3(3Cl-acac)] (waar ttfa = tenoïel trifluoroasetielasetonato, tffa = trifluorofurielasetielasetonato, ntfa = naftieltrifluoro asetielasetonato, btfa = bensoïeltrifluoroasetonato, hfaa = heksafluoroasetielasetonato en 3Cl-acac = 3-chloroasetielasetonato) - bespreek en vergelyk met betrekking tot die intieme geometriese omgewing rondom die niobium(V) metaalkern. Laastens was die diepgaande karakterisering met betrekking tot die vaste toestand tris- en tetrakis- koördineringsmodusse van drie M(V)-tropolonato (Trop) komplekse, [NbO(Trop)3], [Ta(Trop)4Cl] en [Nb(Trop)4Cl], effektief daarin om die verband tussen vastetoestand koördinasievoorkeure en kompleks stabiliteit in oplossing daar te stel. Ten einde die elektroniese omgewing wat deur die niobium(V) en tantaal(V) kerne in hierdie komplekse ervaar word verder te evalueer is ʼn kinetiese studie van die substitusie reaksies van [NbCl6]- en [TaCl6]- met ʼn reeks β-diketone in inkomende ligande onderneem. Die data wat uit hierdie studie verkry is kon sistematies gebruik word om ʼn algehele reaksie meganisme en tempowet af te lei wat alle huidige eksperimentele waarnemings verklaar. Dit is ook gevind dat die eerste term in die ingewikkelde tempowet vereenvoudig kon word na ʼn konstante waarde, gedefinieer deur kfwd, vir ʼn spesifieke ligand teen vasgestelde [Cl-]vry en [H2O]vry waardes. Vanuit die verkrygde resultate en waarnemings aangeteken gedurende studie kan, deur hierdie vergelyking te gebruik, verskeie voorheen onbekende chemiese eienskappe vir hierdie stelsels, soos die bevordering van reaksiesnelhede deur hidrolisering, gekwantifiseer word. Bykomend hiertoe kon ʼn verband tussen die pKa van die ongekoördineerde β- diketonato inkomende ligande en die tempo van ligandsubstitusie ook vasgestel word. Hierdie substitusietempo’s neem merkbaar toe wanneer die pKa van die vry ligand verlaag word. Dit kan toegeskryf word aan die afname in elektrondigtheid by die metaalkern, wat veroorsaak dat die metaal meer vatbaar is vir nukleofiliese aanval. Die vier orde-groottes toename in Brønsted basisiteit van die vry bidentate ligande lei tot ʼn agt en ses maal afname in substitusie reaktiwiteit by die Nb(V) en Ta(V) kerne onderskeidelik. Hierdie ondersoek is afgesluit deur die verskillende eienskappe wat aan niobium(V) en tantaal(V) komplekse gekoppel word te vergelyk, en daardie eienskappe in verband te bring met veranderinge wat in elke kompleks teweeg gebring is deur verskillende koördinerende β-diketonato ligande te gebruik. Verskeie tendense is waargeneem as gevolg van die verskillende elektroniese eienskappe van die onderskeie β-diketonato ligande. Dit was opgemerk dat meer elektronskenkende substituente (hoër pKa) op die acac ruggraat van die kompleks lei tot ʼn beduidende invloed op die oksido, trans effek van die verbindings, asook ʼn skynbare toename in intermolekulêre waterstofbindingsinter-aksies. In teenstelling hiermee word meer elektrondigtheid binne die rand van die β-diketonato ligand aangetref wanneer die elektrononttrekkende vermoë van die substituente op die ligand toeneem. Dit verminder weer die hoeveelheid intermolekulêre waterstof bindingsinteraksies, wat blyk ʼn impak op die sublimasie-eienskappe van die verbindings te hê. Indien al hierdie brokkies inligting saamgevoeg word kan dit dus afgelei word dat verbeterde herwinning van niobium uit sublimasiestudies verwag kan word deur die elektronskenkende vermoë van die β-diketonato substituente te verhoog. Al hierdie waarnemings lei tot beter insigte rakende die eienskappe wat die chemiese en fisiese eienskappe van hierdie stelsels beheer. Die afleidings wat hier bespreek is, is slegs ʼn bewys van die begrip rakende die belangrikheid van ʼn studie om die intieme geometriese natuur en eienskappe van Nb(V)- en Ta(V) koördinering met organiese ligande vir skeidingsdoeleindes toe te lig.