Synthesis and characterization of bis-(salicylaldiminato) chromium and cobalt complexes and their catalytic evaluation for the coupling of CO2/CS2 with propylene oxide

Tumeli, Ts’epo Richard
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The coupling reaction of carbon dioxide (CO2) and epoxides has attracted much attention from researchers. This reaction utilizes CO2, as feedstock, which is relatively non-toxic, cheap, and abundant, to produce biodegradable cyclic compounds or co-polymers. Carbon disulfide (CS2) a more reactive CO2 analogue, can also be utilized in the process to prepare sulfur derivatives which are also commercially relevant .In this investigation, metal complexes prepared from salicylaldimine ligands were prepared and evaluated as catalysts for the coupling of CO2orCS2with propylene oxide (PO). The synthesis and characterization of several salicylaldimine ligands, both neutral and ionic functionalized is described. In addition the synthesis and characterization of neutral and ionic functionalized metal complexes containing Cr(II), Cr(III)and Co(II)metal centers prepared from CrCl2, CrCl3 (THF)3 and Co(OAc)2·4H2O respectively is also reported. Analytical techniques such as FT-IR, UV-Vis and NMR spectroscopy, mass spectrometry, and elemental analysis, where appropriate, were employed as characterization methods for the synthesized ligands and metal complexes. Catalytic evaluation of the synthesized neutral complexes, Cr(II) (1-4), Cr(III) (7-10) and Co(II) (13-16) as catalysts for the coupling of CS2 and PO in neat (solventless) reactions resulted in the production ofonlycyclic products, confirmed with 1HNMR spectroscopy. Preliminary reactions showed that the complexes are not active for the coupling reaction on their own and required the presence of a co-catalyst. Complexes 2 and 8, were the best performing Cr(II) and Cr(III) complexes in the presence of [PPN]Cl as a co-catalyst while the corresponding Co(II) complexes, 14, performed better in the presence of DMAP. The synthesized metal complexes were evaluated in the presence of organic solvents. Significantly better substrate conversions were obtained when the coupling reactions were performed in methanol (MeOH). However, in these reactions different cyclic products were produced compared to products obtained in neat reactions. No clear trend in performance was observed when comparing the Cr(II) and Cr(III) complexes, however it was observed that the Cr(II) complexes performed better than their corresponding Co(II) complexes for reactions performed neat and in MeOH. Product selectivity was observed to be influenced by the nature of the metal when comparing Co(II) and Cr(II) complexes and by the oxidation state when comparing the Cr(II) and Cr(III) complexes. The ionic functionalized complexes, Cr(II) (19 and 20) Cr(III) (21 and 22 and Co(II) (23 and 24) were also evaluated as catalysts for the coupling of CS2 and PO. These complexes were also active producing the same cyclic product observed when reactions are performed neat and in MeOH as solvent. However, in the presence of [PPN]Cl a decrease in product selectivity was observed compared to when the complexes were used on their own. The nature of the metal and oxidation state was also observed to have an effect on catalytic performance. All complexes, both neutral and ionic functionalized, were finally evaluated as catalysts for the coupling of CO2and PO. All reactions carried out produced only propylene carbonate, confirmed with 1HNMR spectroscopy. The complexes showed an increase in PO conversions when the reactions were performed at 80 oC compared to room temperature. When the reaction was performed at 5 MPa CO2pressure, lower conversions of PO was obtained compared to when the reactions was performed at 1 MPa CO2 pressure
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfillment of the requirement for the degree of Doctor of Philosophy (PhD) in Chemistry, 2021