| Summary: | Vanadium is an early first-row transition metal that is known for the beautiful coloured
compounds that it forms in a wide range of oxidation states. In high oxidation states,
vanadium is very oxophilic whilst at low oxidation states, Ï-donating ligands are preferred.
It is the only element in the periodic table to be named after a goddess (the Nordic
goddess Vanadis), and perhaps with this legacy in mind some unpredictable and
surprising chemistry might be expected.
This research study focussed on the rich and diverse coordination chemistry of
vanadium. Various vanadium(IV) and vanadium(V) compounds were successfully
synthesized with O,O and N,O-Bid ligand systems (Bid= five or six membered chelating
ligand via O,Oâ or N,O-donor atoms). These ligands were chosen for their wide
application in terms of industrial use in the development of catalysts as well as their
biological activity for pharmacological application. To achieve the above mentioned aim
various characterization techniques were mastered such as IR, UV/Vis, NMR and single
crystal X-ray diffraction. To this regard four vanadium complexes were successfully
characterized by XRD namely [VO(dbm)2], [VO(dbm)2(MeOH)]â¢2MeOH, [VO(dbm)2py]
and (C9H17O2)[VO2(cupf)2]. The three diketonato containing complexes provided unique
stereo-electronic changes in each case and the effect upon distortion of the vanadium
centre as well as the trans effect of the oxido bond could be evaluated. The last
mentioned compound was of special interest as the novel 2,2,6,6-
tetramethyldihydropyran-4-onium that acts as cation for the anionic vanadium complex
was speculated to have formed either by cyclization of acetone during the reaction or by
action of the vanadium present.
In addition to the synthesis component of the research a kinetic substitution study was
instigated. The complex solution chemistry of vanadium resulted in a wide array of
experiments to evaluate the effects of not only ligand concentration on reaction rates but
also pH dependence of certain species in solution. This culminated in a proposed
reaction mechanism and rate law that accounts for various pH, pKa and concentration
effects. As vanadium is known for its biological activity, selected complexes synthesized from this
study was investigated for in vitro cancer screening. These results were concluded as not
being positive but provided valuable insight for future ligand and complex design.
51V NMR was effectively used in this study both in the synthesis component as well as
the kinetic study conducted. Valuable insight into the electronic environment experienced
by the vanadium centre was obtained and correlations could be established between
steric strain within a complex and the amount of shielding experienced by the vanadium
centre. Additionally, experiments such as in the kinetic study could be followed over time
on 51V NMR and revealed important information regarding product formation and the
identification of an intermediate [VO(O2)(2,3-dipic)]2- in the reaction which was vital in
construction of the reaction mechanism.
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