The first work package was dealing with the synthesis of bimetallic complexes.
The work began with a thorough literature review to establish a solid foundation and formulate an experimental plan. Synthetic procedures were then initiated, leading to the successful synthesis of ligands and their subsequent use in the preparation of bimetallic complexes. All synthesised compounds were fully characterised to confirm their structures and properties.Four homobimetallic complexes without linkers were synthesised to facilitate studies under homogeneous conditions. A novel approach was developed to append a pyrene moiety to the ligand using an ether linker, and this ligand was successfully metalated with cobalt, iron, nickel, and copper. A heterobimetallic Cu/Fe complex based on an N2S2 bpy ligand was also synthesised. This complex exhibited intriguing fluxional properties, warranting extensive characterisation. In response to findings from WP2, a new bimetallic ligand scaffold incorporating an amine group was synthesised to enhance CO2 binding capabilities.
The second work package explored the reactivity of the synthesized complexes towards CO2 and the study of the related mechanism
This was assessed using standard electrochemical techniques such as cyclic voltammetry and controlled potential electrolysis. Special focus was given to cobalt complexes to enable comparisons with previously studied monometallic catalysts. Bulk electrolysis was performed, and the products were analysed using gas chromatography and ion chromatography. Mechanistic investigations were carried out using UV-vis spectroelectrochemistry and attempts to synthesise hydride complexes.
All complexes demonstrated measurable reactivity towards CO2 during cyclic voltammetry studies. However, cobalt complexes were prioritised for bulk electrolysis experiments. The electrolysis revealed no C2+ products, and challenges were encountered in detecting liquid products in organic electrolytes. The preference of the bimetallic complexes for proton reduction over CO2 reduction prompted the redesign of the ligand scaffold, incorporating an amine group as a linker. For the Cu/Fe complex, the reduced Cu(I) species, a likely catalytic intermediate, was successfully isolated and characterised using EPR, 1H^1H1H NMR, and IR spectroscopy. Preliminary experiments on this complex with CO2 showed some activity, though further investigations were limited due to the early termination of the project.
The third work package consisted in the immobilisation of the pyrene-appended complexes synthesised in WP1 on carbon electrodes through non-covalent interactions. The carbon nanotubes were drop-cast onto electrodes and incubated with solutions of the catalysts. Both bimetallic and homometallic complexes bearing the pyrene group were immobilised and evaluated. The immobilisation process was successful, and comparative studies were conducted to evaluate CO2 reduction on the electrode surface. Turn Over Numbers (TON) were significantly higher for immobilised complexes than for those in homogeneous conditions. The monometallic complexes were more active for CO2 reduction, producing primarily CO, while the bimetallic complexes favoured hydrogen evolution. Attempts to study the reaction mechanism using surface spectroelectrochemical IR were made, though these proved to be technically challenging.
Despite its early termination, this research project demonstrated the successful synthesis of innovative bimetallic complexes, explored their reactivity towards CO2, and advanced the study of their immobilisation on electrode surfaces. The findings provide a foundation for future studies on improving CO2 reduction catalysts and understanding their mechanisms.
