Nucleophile assisted carbon dioxide fixation for a cleaner environment

[摘要] 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.

[发布机构] University of the Free State