Professor Mu-Hyun Baik of Indiana University is supported by the Chemical Catalysis Program in the Division of Chemistry to develop a rational design for the synthesis of new organometallic catalysts for CO2 reduction. A series of integrated computational and experimental studies are proposed that aim at delineating the mechanism of CO2 activation with Tanaka's [Ru(tpy)(bpy)(CO)]2+ catalytic system.
The results will lay the foundation for the systematic development of new catalytic systems with cheap Fe/Co centers as a viable substitute for expensive ruthenium-based catalysts for CO2 reduction to commercially-important starting materials. A Graduate students and a postdoc will be trained in integrated theoretical and experimental approaches towards the design of practical catalytic systems.
The aim of this project was to identify a strategy for designing a catalyst that can generate valuable chemicals from carbon dioxide. Utilizing CO2 for chemical sythesis is an important goal for future green technologies. Currently the most common primary product of CO2 reduction catalysis is formic acid, which is not a desirable product and must be further processed to afford technically useful products. These followup reactions are challenging and no technical solutions exist currently, that would tranform formic acid into a more useful product. The biggest challenge lies in increasing the size of the product to molecules that contain C-C bonds. We began our work by examining one of the very few catalysts known to produce C-C coupled products in trace amounts during electrocatalytic reduction of CO2. Tanaka's complex Ru(bpy)(tpy)(CO)2+ was observed to produce very small amounts of oxalic acid during catalysis. Combining theoretical and experimental techniques, we determined that Ru(bpy)(tpy)(CO)2+ is not the catalytically active compound. Instead the bpy ligand must be cleaved upon reduction to afford the catalytically competent metal complex Ru(tpy)(CO)0. We prepared the Ru(tpy)(CO) fragment independently and were able to show that it displays the same catalytic activity as the original compound. Our work revealed several new concepts about how to reduce CO2 and form C-C bond simulataneously suggesting a potential strategy towards designing new C-C coupling catalysts: (1) A straightforward sequence of steps that can be envisioned for CO2 reduction catalysis coupled to C-C bond formation is shown in Figure 1. The catalytic center may perform CO2 reduction twice and promote C-C coupling subsequently. Our analysis revealed not only that the Rh(tpy)(CO) catalyst functions differently, but also that this simplistic pathways is impossible: Upon reduction, CO2 becomes a negatively charge carboxylate anion. The Coulobic repulsion between the two carboxylates are simply too great to allow any favorable interactions. (2) We discovered that a much more viable pathway to forming C-C coupled products is to reduce the first equivalent of CO2 to carboxylate, but transform the second equivalent of CO2 to another common CO2 reduction product, CO. This can be accomplished by consuming one more equivalent of CO2 and form the carbonate antion. Carobn monoxide and carboxylate are much more compatible with each other for C-C coupling reactions. (3) This insight highlights why C-C coupled products are so rare - for a catalyst to perform this task, it has to promote two very different CO2 reduction catalysis. One affording carboxylate and the other affording CO as the final product. It is difficult to envision how a single metal center can promote both reaction steps efficiently. (4) Our atomistic understanding of the CO2 reduction catalysis mechanism suggests that a viable strategy towards catalyst design is to place two reactive sites, one for generating carboxylate and the other for generating carbon monoxide from CO2 next to each other. This design allows for optimizing the performance of each of the two centers independently and avoid the electrostatic barrier of the carboxylate-carboxylate coupling.
