Nucleophile assisted carbon dioxide fixation for a cleaner environment
| dc.contributor.advisor | Roodt, Andreas | |
| dc.contributor.author | Redgard, Shaun | |
| dc.date.accessioned | 2019-07-15T06:31:52Z | |
| dc.date.available | 2019-07-15T06:31:52Z | |
| dc.date.issued | 2019-02 | |
| dc.description.abstract | English: Nature has perfected CO2-fixation in plants through the C3, C4 and CAM (crassulacean acid metabolism) mechanisms. Thus, by applying a biomimetic approach to CO2-fixation the knowledge and approach can be ameliorated. This led to the identification of four “non- nucleophilic” bases, which can be categorized as amidines or guanidines, that have an innate ability to coordinate to CO2. The amidines were 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5- diazabicyclo[4.3.0]non-5-ene (DBN) and the guanidines were 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,1,3,3-tetramethylguanidine (TMG). Overall, the main pursuit of this study was to elucidate on the relevant aspects pertaining to the assimilation and activation pathways of CO2. This was performed by firstly confirming the coordination ability of the bases to CO2 through preliminary solution studies, which indicated that TBD had the strongest ability followed by DBU, DBN and TMG. Thereafter, two model complexes were identified in literature that contained the bases. The general formulae for the two model complexes that were synthesized and characterized were trans-[Pd(L)2Cl2] and [Rh(L)(COD)Cl] (L = (DBU, DBN, TMG, TBD)), except for the TBD-Rh(I) complex. Of the complexes synthesized, three yielded single crystals, of which two were novel complexes, that were suitable for single-crystal X-ray diffraction (SC-XRD); namely trans-[Pd(DBN)2Cl2], [Rh(COD)(DBU)Cl] and [Rh(COD)(TMG)Cl]. The novel Pd(II) complex packed centrosymmetrically, while the Rh(I) complexes were evaluated on the influence the bases had on the 1,5-cyclooctadiene (COD) conformation by assessing three dihedral angles within the COD. Of the three angles, the most significant difference is seen in the jaw angle (ψ) – between the two complexes (ψ = 75.3(3)° and 64.7(4)° for the DBU and TMG complexes respectively). This was attributed to increased electron density in the π antibonding orbitals on the metal centre, for the latter complex, which resulted in an increase in steric hindrance from the metal centre towards the back-bones of the COD. Therefore, in theory, substitution reactions of the bases by other strong bases would lead to a faster reaction in the TMG-containing complex as opposed to the DBU- Rh(I) complex. This is due to increased reactivity from the two electron donating pathways (σ and π donation) to the metal centre aiding the π back-donation to the diolefin. This notion was confirmed through extremely fast substitution kinetic reactions observed in the two Rh(I) complexes by 4-dimethylaminopyridine (DMAP) at different temperatures, because the leaving group (being DBU or TMG) determines the rate of attack based on its electron density contribution to the metal centre. Furthermore, the substitution reaction followed a typical and classical associative mechanism and was supported by the negative ΔS≠ value determined for both complexes. The forward rate constant k1 was ten times slower and ca. 3 % less entropy driven for the DBU-complex than for the TMG-complex, with neither experiencing a strong solvation/reverse pathway. Thus, similar rates may be achieved with CO2 but the rate being limited by the initial activation of CO2 by the bases. Additionally, the large solvent pathway may add to the reaction by performing the reaction under supercritical CO2. | en_ZA |
| dc.description.abstract | Afrikaans: Die natuur het CO2-fiksering in plante deur middel van die C3, C4 en CAM (“Crassulacean”- suurmetabolisme) meganismes vervolmaak. Dus, deur die toepassing van 'n biomimetiese benadering tot CO2-fiksering kan die fundamentele kennis uitgebrei word. Dit het gelei tot die identifisering van vier "nie-nukleofiele" basisse, wat as amidiene of guanidiene gekategoriseer kan word, wat 'n inherente vermoë het om CO2 te koördineer. Die amidiene was 1,8- diazadisiklo[5.4.0]undek-7-een (DBU) en 1,5-diazadisiklo[4.3.0]non-5-een (DBN) en die guanidiene was 1,5,7-triazadisiklo[4.4.0]dek-5-een (TBD), 1,1,3,3-tetrametielguanidien (TMG). Die hoofdoel van hierdie studie om relevante aspekte rakende die assimilasie- en aktiveringsbane van CO2 te ondersoek. Dit is bewerkstellig deur eerstens die koördinasievermoë van die basisse aan CO2 te evalueer deur middel van voorlopige oplossingstudies, wat daarop dui dat TBD die sterkste vermoë het, gevolg deur DBU, DBN en TMG. Daarna is twee modelkomplekse in die literatuur geïdentifiseer wat die basisse bevat. Die algemene formules vir die twee modelkomplekse wat gesintetiseer en gekarakteriseer, was trans-[Pd(L)2Cl2] en [Rh(L)(COD)Cl] (L = (DBU, DBN, TMG, TBD)). Van die komplekse wat gesintetiseer is, het drie enkelkristalle gelewer, waarvan twee nuwe komplekse was en geskik vir enkel-kristal X-straaldiffraksie; naamlik trans-[Pd(DBN)2Cl2], [Rh(COD)(DBU)Cl] en [Rh(COD)(TMG)Cl]. Die nuwe Pd(II) kompleks het sentrosymmetries gepak. Die Rh(I) komplekse is geëvalueer deur middel van die invloed wat die basisse op die konformasie van 1,5-siklooktadieen (COD) gehad het, soos gemanifesteer deur drie hoeke binne die COD. Vanuit hierdie drie hoeke is die belangrikste verskil in die kaakhoek (Eng. Jaw angle) (ψ) waargeneem - tussen die twee komplekse (ψ = 75.3 (3) ° en 64.7 (4) ° vir die DBU en TMG komplekse onderskeidelik). Dit is toegeskryf aan verhoogde elektrondigtheid in die π- antibindingsorbitale op die metalsentrum, vir laasgenoemde kompleks, wat gelei het tot 'n toename in steriese interaksie met die ruggraat van die COD. Gevolglik in teorie, sal substitusiereaksies van hierdie basisse deur ander sterk nukleofiele lei tot 'n vinniger reaksie in die TMG-bevattende kompleks in teenstelling met die DBU-Rh(I) kompleks. Dit is te wyte aan verhoogde reaktiwiteit van die twee elektrondonerende interaksies (σ en π) aan die metalsentrum wat die π terugdonering aan die diolefin help. Hierdie is bevestig deur middel van baie vinnige substitusiekinetiese reaksies wat by verskillende temperature in die twee Rh(I) komplekse deur 4-dimetielaminopiridien (DMAP) waargeneem word, omdat die verlatende groep (DBU of TMG) die tempo van aanval bepaal op grond van die elektrondigtheidsbydrae tot die metaal sentrum. Verder het die substitusiereaksie 'n tipiese en klassieke assosiatiewe meganisme gevolg soos afgelei van die negatiewe ΔS≠ waarde wat vir beide komplekse waargeneem is. Die voorwaartse tempokonstante k1 was tien keer stadiger en ongeveer 3% minder entropie gedrewe vir die DBU-kompleks in vergelyking met die TMG-kompleks, en het nie 'n sterk oplosmiddel/ terugwaartse getoon nie. Dus, soortgelyke tempos kan met CO2 behaal word, maar die relatiewe tempo sal afhang van die aanvanklike aktivering van CO2 deur die basisse. Daarbenewens kan die groot oplosmiddelpad bydrae tot die reaksie, deur dit onder superkritiese CO2 uit te voer. | en_ZA |
| dc.description.sponsorship | SASOL | en_ZA |
| dc.description.sponsorship | University of the Free State | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/10131 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Dissertation (M.Sc. (Chemistry))--University of the Free State, 2019 | en_ZA |
| dc.subject | CO2 | en_ZA |
| dc.subject | DBU | en_ZA |
| dc.subject | DBN | en_ZA |
| dc.subject | TMG | en_ZA |
| dc.subject | TBD | en_ZA |
| dc.subject | DMAP | en_ZA |
| dc.subject | COD | en_ZA |
| dc.subject | X-ray diffraction | en_ZA |
| dc.subject | Kinetic reactions | en_ZA |
| dc.title | Nucleophile assisted carbon dioxide fixation for a cleaner environment | en_ZA |
| dc.type | Dissertation | en_ZA |