Electro- and Photochemical Activities of Manganese-Based Complexes for CO2 Reduction

Abstract

Effective CO2 reduction is a grand challenge in modern chemistry. One promising approach is via electrochemical or photochemical catalysis using molecular catalysts. Among the available options, Mn-based complexes are an emerging class of earth-abundant molecular catalysts. In this thesis, we investigated the electro- and/or photo-chemical activities of several Mn-based bipyridyl carbonyl complexes for CO2 reduction. This thesis is divided into three parts: (i) a case study of the electro- and photochemical activities of a CN-bridged binuclear Mn complex; (ii) a photophysical investigation of the CN-bridged complex; (iii) a mechanistic study of the key role played by an Mn tetracarbonyl complex in electrochemical CO2 reduction. The first part of this work was motivated by an undesirable photodimerization observed in the reaction of a benchmark [Mn(bpy)(CO)3Br] upon exposure to UV and visible light, thereby hindering the activity of this benchmark complex as a photocatalyst for CO2 reduction. To overcome this undesirable photodimerization, we proposed and characterized the cyanide-bridged complex, and investigated its activity for electrochemical CO2 reduction. The CN-bridged complex indeed inhibited the dimerization upon photolysis and carried out a photochemical CO2 reduction. In the second part of this work, we further studied the CN-bridged complex to decipher the photochemical reaction. To do so, we utilized infrared and transient spectroscopy to identify the photo-induced intermediate and the corresponding photophysics. Based on this analysis, we found that the CN-bridged complex exhibits structural robustness, which prevents the cleavage of the CN-linkage, leading to the undesirable photodegradation under photochemical conditions. Furthermore, we identified that solvent participation influences the excited state dynamic, i.e., a relatively long-lived intermediate in the coordinating solvent. In the third part of this work, we focused on understanding the mechanism behind the electrochemical CO2 reduction of the benchmark, [Mn(bpy)(CO)3Br]. In particular, we synthesized and characterized an [Mn(bpy)(CO)4]+ complex, which has been suggested theoretically in the literature as an intermediate. With the aid of infrared and in situ UV-Vis spectroelectrochemical studies, we verified the key role played by the tetracarbonyl complex—it serves as both a precatalyst and an on-cycle intermediate—in the Mn-based electrocatalytic CO2 reduction.