Establishing Structure-Function Relationships for the Catalytic Reduction of Carbon Dioxide with Chromium-Based Molecular Catalysts with Redox Mediators

As the atmospheric concentration of carbon dioxide (CO2) continues to rise, new strategies for the conversion of waste CO2 to value-added products are of the upmost importance. While the electrocatalytic reduction of CO2 has been studied for several decades, improvements are still needed for implementation on an industrial scale. First-row transition metal catalysts are of interest due to their lower cost and high abundance on Earth compared to metals such as platinum. Until the last several years, no catalysts with a chromium (Cr) metal center capable of the CO2 reduction reaction (CO2RR) with quantifiable efficiencies had been reported. The first Cr-centered catalyst for the CO2RR contained a redox-active 2,2′-bipyridine-based N2O2 ligand framework. To understand the structure-function relationship of the ligand framework on catalysis, four additional Cr-centered catalysts are discussed with modified bpy-based N2O2 or terpyridine-based N3O ligand frameworks. All of the catalysts are quantitatively selective for the reduction of CO2 to carbon monoxide (CO) and the most active of the series has a turnover frequency (TOF) of 9.29 s–1.
The development of co-electrocatalytic systems utilizing redox mediators (RMs) is of growing interest as a general strategy for the improvement of electrocatalytic small molecule conversion. RMs assist in the transfer of electron equivalents from the electrode to the catalytic active site providing a kinetic improvement to the overall reaction. A series of dibenzothiophene-5,5-dioxide (DBTD)-based RMs are investigated with the Cr-based complexes to provide a foundational understanding of the co-catalytic activity and mechanism. All co-catalytic systems maintain selectivity for CO and enhance the TOF of the parent catalyst by up to 142-fold. The RMs are proposed to operate via an inner-sphere electron transfer mechanism where the RM formally binds to the Cr center during co-catalysis. This key intermediate is stabilized by a pancake bonding (PB) interaction between the redox-active ligand backbone and the RM, which is also aided through dispersion interactions and a dative covalent interaction between the sulfone and Cr center. Through the analysis of electroanalytical experimental data and computational studies, the kinetic and thermodynamic properties which enhance or weaken the PB interaction are evaluated through iterative synthetic modification.