Abstract:
The design of new catalysts with high catalytic efficiency and robustness towards carbon dioxide (CO2) reduction is of paramount importance. A full understanding of the requirements for creating a catalyst of this type is a missing gap in the knowledge base which prevents progress in synthesizing solar fuels. Our research hypothesis is that through the control of steric and electronic factors we can design better catalysts using various transition metals. Multidentate ligands composed of pyridyl bound to N-heterocyclic carbene rings are a synthetically flexible scaffold capable of testing our hypotheses. The systematic tuning of this scaffold will elucidate the factors necessary to improve active catalysts and extend our results to more complex systems. These ligand motifs are also commonly seen in many active catalysts for CO2 reduction. Therefore, the systemic investigation of the properties that lead to higher activity in scaffolds containing these motifs can concurrently suggest improvements for many systems already available.Reduction catalysts, supported by a CNC-pincer moiety, are some of the most robust catalysts in the literature while maintaining good catalytic activity. The CNC-pincer scaffold have shown tremendous results with electronic tunability of the pyridyl N atom through para substitution of the pyridinol ring. A ruthenium(II) catalyst utilizing the CNC pincer has shown 250 turn-over numbers of carbon monoxide (CO) over 40 h with a catalyst loading of 100 µM. The cobalt(I) systems are slower; however, they are even more robust than the ruthenium analog producing 203 TON of CO2 over 72 h with a catalyst loading of 1 µM. Notably, the cobalt catalyst utilizes an inexpensive, less toxic, and earth abundant metal center compared to ruthenium. Another scaffold, with NCCN donors binding in a tetradentate fashion, is currently being investigated with nickel(II) and cobalt(II) metal centers. These studies are producing a better understanding of the catalyst structure needs for a solar to chemical fuel catalyst to enable a carbon-neutral fuel cycle within our current fuel infrastructure. This catalytic system will constitute an artificial photosynthesis scheme by producing fuels and other useful chemicals from carbon dioxide, mimicking how plants store solar energy in glucose.
