$CO_2$ conversion using an iron complex supported by an imino-tris(phenolate) ligand and a nickel complex supported by an acridane-based amido diphosphine pincer ligand

Abstract

Carbon dioxide conversion into valuable chemicals has received much attention as $CO_2$ is an abundant, nontoxic and renewable C1 source. The most promising way to exploit $CO_2$ is through coupling reactions with various epoxides to produce poly- or cyclic carbonates. In Chapter 1, optimization for the selective formation of cyclic carbonate at a pre-organized single iron center will be presented. Cyclic carbonates have been used as precursors for polycarbonate, electrolytes and intermediates in organic synthesis. In order to achieve selective and effective conversion of $CO_2$ to cyclic carbonate with high TOFs, we designed a new ligand scaffold that can accommodate an iron center in a three-fold symmetry. To the best of our knowledge, the designed iron catalyst shows the highest known TOF for the coupling reaction of $CO_2$ and propylene oxide. Notably, it is suggested that $CO_2$ is involved in the rate-determining step for generating a six-coordinate transition state species. This result supports the notion that the pre-organized structure significantly lowers the activation barrier of $CO_2$ insertion. Iron(III) complexes for a model study were prepared and crystal structures of both five- and six-coordinated complexes were obtained. In order to achieve a high catalytic performance with ${($NO_3$)Fe(THF)}_2$, reaction conditions were optimized with several variables including pressure, temperature and co-catalyst/catalyst ratio. Under the optimized conditions, ${($NO_3$)Fe(THF)}_2$ showed an unusual catalytic efficiency with propylene oxide. DFT calculations were performed to obtain energy profiles, which indicated that the $NO_3$ ligand significantly stabilizes the transition state of the formation of cyclic carbonate by facilitating the formation of an octahedral geometry with two oxygen atoms each from $CO_2$ and an epoxide accommodated in a cis fashion. Further investigation is currently in progress in order to reveal a correlation between the geometry of the catalyst and the reactivity.

Despite remarkable advances in $CO_2$ reduction chemistry using transition metal complexes, achieving high efficiencies and selectivity in $CO_2$ conversion remains a practical task. In nature, carbon monoxide dehydrogenase (CODH) shows a high catalytic activity for the reduction of $CO_2$ to CO. In Chapter 2, inspired by the biological enzyme, $CO_2$ conversion was investigated with nickel species supported by PNP ligands. In the original PNP system, the selective conversion of $CO_2$ did not solely occur at a nickel center. However, in the $^{acri}PNP$ system, $CO_2$ was converted to CO with the yield of 75% at a single nickel center. Although CO2 conversion was achieved with a fairly high yield, there were side products including ($^{acri}PN^HP$)Ni$(CO)_2$ indicating there is room for improvement. According to the previous study, a reduction in the nucleophilicity of the central nitrogen may play an important role in selectivity. Thus, we will explore how to reduce the nuceleophilicity of the central nitrogen atom. Computational studies reveal that a steric effect can be proposed as a major factor to reduce the nucleophilicity of the central amide moiety. According to the space filling models of the optimized geometries of modified $^{acri}PNP$ ligands, relatively bulkier groups could efficiently protect the central nitrogen atom to reduce its nucleophilicity. To evaluate the postulation that a steric effect may be the key factor in reducing the nucleophilicity of the central nitrogen atom, $CO_2$ conversion reaction with Ni(0) complexes with modified ligands will be conducted.