Inorganic Research

1. Nanomaterials: Metal Organic Framework materials. (Professor GM Watkins).  g.watkins@ru.ac.za

Metal-organic molecular frameworks (MOFs) may be formed in the crystalline phase by the self assembly of structurally defined molecular modules possessing translational symmetry of one or more dimensionality. Such open framed network materials have a wide variety of structural diversity and are expected to exhibit novel properties such as inclusion behaviour, and are of interest as potential hydrogen storage devices. We are presently examining MOFs based on 1,2,4,5-benzenetetracarboxylic acid (H4B4C).

2. Bioinorganic metal complexes. (Professor GM Watkins).

Complexes of Schiff bases have interesting catalytic and biological activities. We are presently examining the synthesis of various Schiff base complexes and testing their biological activity using the brine shrimp (Artemia salina) lethality assay, and testing their in vitro growth inhibitory activity against pathogenic bacteria, namely Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and the fungus, Aspergillus niger using the disk diffusion method.

3. Ligand isotope studies of transition metal complexes (Professor GM Watkins).

Vibrational spectra of metal complexes are best assigned using a variety of empirical and modelling techniques. We have employed ligand isotope studies (eg. 13CO; C2D4; py-d5O; quin18O) of the infrared and Raman spectra of metal complexes to address various assignment issues, coupled with molecular modelling calculations performed with semi-empirical and DFT methods.

4. Vanadium anti-diabetic pharmaceuticals (Dr ZR Tshentu). z.tshentu@ru.ac.za

The vanadium compounds are active in lowering the glucose levels, by oral administration of either vanadium salts (e.g., VOSO4 or sodium vanadate) or organically chelated V(III), V(IV), or V(V) complexes, and this makes them promising candidates for the treatment of type 2 diabetes. The recent successes achieved with organo-vanadium complexes suggest that modification of the metal ion chemistries by the organic ligands not only increased efficacy but also decreased toxicity compared with the inorganic vanadium salts and other available prescription drugs. The oral hypoglycaemic agents in the clinic, such as sulphonylureas (eg Glipizide), biguanides (eg Metformin) and meglitinides (eg Prandin present side effects including, hypoglycaemia and weight gain for sulphonylureas, lactic acidosis for the biguanides, etc., therefore, there is a need for the development of oral agents that do not present these side effects.

There are three phases to this project, namely, (i) the syntheses of the biologically active organic ligands and their respective vanadium complexes, (ii) the thermodynamic stability studies using potentiometry and the program Hyperquad to determine the stability constants and therefore the species distribution of these complexes as a function of pH, and (iii) the biological studies (in collaboration with Dr C. Frost of NMMU biochemistry) using INS-1 pancreatic cell lines to investigate the insulin activation potential of these complexes.

5. Heterogeneous vanadium catalysts for oxidation reactions (Dr ZR Tshentu).

Oxidation processes are particularly important for the conversion of the by-products of the Fischer-Tropsch synthesis such as unsaturated hydrocarbons, alcohols and other oxygenated products. Oxidation of olefins into useful products is of immense interest. For example, epoxides are useful intermediates, obtained on catalytic oxidation of alkenes, that are used for petrochemicals, fine chemicals, and production of epoxy resins.

Transition-metal-catalyzed oxidation of organic compounds with atom efficient oxidants such as O2 or H2O2 is rapidly gaining importance as a viable alternative to the environmentally hazardous and toxic oxidising agents such as permanganates and dichromates. The project entails the synthesis of polymer-anchored oxovanadium(IV) complexes and the evaluation of their catalytic activity. Amongst the important chemical transformations that will be studied is the following:

(a) Vanadium-catalyzed oxidation of olefins (styrene will be used as a substrate) to epoxides using hydrogen peroxide,
(b)Vanadium-catalyzed C-H oxidation of alkyl benzene with hydrogen peroxide,
(c) Oxidation of sulfides (using methyl phenyl sulfide as a substrate).

6. Radiopharmaceutical development-Bifunctional chelates for the Re(I) tricarbonyl core (Dr ZR Tshentu).

The widespread interest in the development of rhenium radiopharmaceuticals is a direct result of their ideal nuclear properties (strong ?-emission energies of 1070 keV and 2120 keV, and long half-lives of 90 h and 17 h for 186Re and 188Re, respectively) which are suitable for targeted radiotherapy of cancers. The recent development of the [Re(CO)3]+ core has garnered significant interest because of the easy availability of the [Re(H2O)3(CO)3]+ precursor in aqueous conditions.

Tridentate imidazole-containing ligands with appended carbohydrate moieties (see the structures below) and the subsequent synthesis of the Re(I) complexes via the substitution of the three labile water molecules of the precursor compound with these ligands will be synthesized. The appended moieties, additional to carbohydrates, could also include peptides and steroids which would act as the biologically active groups for targeting specific tissue while the chelate delivers the radioactive atom within the locality of the target site.

The complexes will be fully characterized using elemental analysis, infrared spectroscopy, 1H NMR and single crystal X-ray crystallography. The biodistribution pattern of the technetium analogues of the rhenium complexes may be investigated (in an attempt to gain knowledge of their accumulation patterns in the body). The kinetics of complexation will be investigated in collaboration with Dr T. Mtshali of the University of Free State.

7. Liquid-liquid extraction process for the separation of base metals (Dr ZR Tshentu).

Processes to improve ore dissolution and metal ion separation are in continuous demand, in view of their economical implications in today’s competitive world. The idea of bioleaching (the extraction of metals from their ores involving sulfur oxidizing bacteria, such as Acidithiobacillus thiooxidans) of the later 3d transition metals from their ores presents a relatively unexplored route for the separation of the base metals via a solvent extraction process using suitable ligands in a sulfate medium.

The objective of this study is to achieve nickel separation from the bioleach solution of the later 3d transition metals via a liquid-liquid extraction process using suitable ligands. The metallic ions in this solution are Cu2+, Zn2+, Mn2+, Co2+, Fe3+ and Ni2+, with the latter in large excess and Fe3+ as a major contaminant. The sulfate ions do not phase-transfer readily due to their high hydration energy, therefore, suitable anions which are compatible with the organic phase will be included in this study to ion pair the cationic complexes to achieve their extraction into the organic phase possibly resulting in a synergistic extraction process.

The nature of the extracted species, using both solid state and solution studies will be determined using the usual characterization techniques such as infrared spectroscopy, electronic spectroscopy, magnetic and conductivity measurements as well as single crystal X-ray crystallography. The solution chemistry of these complexes in aqueous phase will be thoroughly investigated by the evaluation of the protonation and formation constants by the competitive ion method using potentiometry and Hyperquad. A possibility of carrying out the potentiometric acid-base titrations in a two-phase system also exist in order to approximate more closely the complexation thermodynamics of the extraction process. This will be necessary since the formation constants are an important factor for the successful separation of the later divalent 3d metals.