Rhenium and Manganese α-diimine tricarbonyls as CO2 reduction catalysts: Insights from novel ligand design

Concerns over climate change and energy security combined with the commercial application of carbon capture technologies has led to increased interest in the use of CO2 reduction catalysts as a means to convert this captured waste into fuel. In order to accomplish this photocatalytic or electrocatalytic systems must be employed of which the Lehn type catalyst based on the original [ReCl(CO)3(bpy)] complex is highly suitable due its selective CO formation and high efficiency. The purpose of this project was to develop new understanding of Lehn type CO2 reduction catalysis and, in particular, to develop new families of catalysts integrating manganese in place of rhenium. Much of the research has focussed on the concept of decoupling – either decoupling electron withdrawing groups from the α-diimine or decoupling the electronic effects from the steric effects. A variety of rhenium and manganese complexes have been synthesised and studied using a variety electrochemical and spectroscopic methods. Initial research focussed upon developing our understanding of the photodecomposition of the manganese based Lehn type catalysts and it was determined that the complexes decay via CO elimination giving a large variety of decomposition products. This research led to the investigation of the electrocatalytic and photocatalytic properties of three rhenium bis(mestiylimino)-acenaphthene complexes which exhibited electrocatalytic activity but not photocatalytic activity. The major family of ligands studied were asymmetric imino pyridine ligands which due to the break of symmetry between the phenyl moiety and the diimine allow for sterically demanding groups to be incorporated into the complexes without changing the electronic properties of the complex. These complexes are ideal for ‘lab mouse’ investigations of systems that show sensitivity to both steric and electronic factors. It was observed that while the rhenium imino pyridine complexes behaved in a manner similar to bipyridine complexes the manganese variants exhibited behaviour more akin to what has been observed in manganese diazabutadiene catalysts. Attempts to provide quantitative analysis of catalyst performance led to the employment of many different techniques ranging from gas chromatography to line shape analysis of voltammograms, however, no satisfactory method of performing quantitative analysis could be found. The overall conclusion is that the manganese based Lehn type catalysts can be used effectively as homogeneous electrocatalysts but the photosensitivity prohibits practical use in photocatalytic systems. The asymmetric imino pyridine ligands have shown great potential for systematic investigation of CO2 reduction catalysts and offer enormous scope for further development, however it is necessary for the community to adopt and publicise standards for benchmarking new catalysts as the methods employed today are not ideal.