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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-05172013-152607 |
| Date | 17 May 2013 |
| Creators | Herbst, Leandra |
| Contributors | Prof HG Visser, Prof A Roodt |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-05172013-152607/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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