A SOLUTION AND SOLID STATE STUDY OF NIOBIUM COMPLEXES

• Main Author: Herbst, Leandra
• Other Authors: Prof HG Visser
• Format: Others
• Language: en-uk
• Published: University of the Free State 2013
• Subjects: Chemistry
• Online Access: http://etd.uovs.ac.za//theses/available/etd-05172013-152607/restricted/

This research project focused on the investigation and identification of various niobium(V) complexes containing selected O,Oâ-bidentate ligands that could potentially be used for the selective separation of niobium from tantalum. Emphasis was placed on acetylacetone (acacH) type of ligands due to the ease of varying their electronic and steric properties. The crystallographic characterization of three novel complexes, (acetylacetonato-κ2- O,Oâ)chloridotrimethoxidoniobium(V) (1), the âcageâ-like structure of tetrakis- (acetylacetonato-κ2-O,Oâ)octakis(etoxy)tetrakis(μ2-oxo)tetraniobium(V) (2) and the two structures that were obtained from the same crystal, (1-phenyl-1,3- butanedionato-κ2-O,Oâ)chloridotrimethoxidoniobium(V) (3a) and (1-phenyl-1,3- butanedionato-κ2-O,Oâ)dichloridodimethoxidoniobium(V) (3b), is discussed and compared to literature. Complex 1 crystallized in an orthorhombic crystal system and space group Pbca, while complexes 2, 3a and 3b all crystallized in a monoclinic crystal system and a space group P21/c, for all. In general it was observed that these mono substituted β-diketonato complexes of niobium(V) crystallized in a distorted octahedral coordination polyhedron. The average O-Nb-O bite angle and Nb-O bond distance for these complexes were determined as 80.5 (1) ° and 2.108 (2) Ã, respectively. A kinetic investigation was conducted to follow the formation of the (acetylacetonato- κ2-O,Oâ)chloridotrimethoxidoniobium(V) complex in methanol. The coordination mechanism is postulated for the two observed steps of acacH coordination, of which the initial coordination of the ligand takes place in the first step. The equilibrium constant, K1, was determined as 1975 (201) M-1 at 25.0 °C. The second, rate determining step is representative of the total reaction and includes the ring-closure of the acac ligand and yields K1 as 1403 (379) M-1. Within experimental error, this value is in good agreement with that of the first step. When comparing the rate constants, k1 and k2, it is found that the first reaction is roughly six orders of magnitude (106) faster than the slower, second reaction step. 93Nb NMR was successfully used in characterising the niobium(V) products synthesised and played an important role in the kinetic study of the project. With regards to the kinetic study; solvent coordination proceeded rapidly upon solvation of the dimeric starting material, [NbCl5]2, in methanol and the niobium(V) starting reagent was correctly determined as [NbCl2(OMe)3(MeOH)] through 93Nb NMR.