Abstract
The selective hydrosilylation of carbon dioxide (CO2) to either the formic acid, formaldehyde, or methanol level using a molecular cobalt(II) triazine complex can be controlled based on reaction parameters such as temperature, CO2 pressure, and concentration. Here, we rationalize the catalytic mechanism that enables the selective arrival at each product platform. Key reactive intermediates were prepared and spectroscopically characterized, while the catalytic mechanism and the energy profile were analyzed with density functional theory (DFT) methods and microkinetic modeling. It transpired that the stepwise reduction of CO2 involves three consecutive catalytic cycles, including the same cobalt(I) triazine hydride complex as the active species. The increasing kinetic barriers associated with each reduction step and the competing hydride transfer steps in the three cycles corroborate the strong influence of the catalyst environment on the product selectivity. The fundamental mechanistic insights provide a consistent description of the catalytic system and rationalize, in particular, the experimentally verified opportunity to steer the reaction toward the formaldehyde product as the chemically most challenging reduction level.
Highlights
The catalytic reduction of carbon dioxide (CO2) to valueadded products is essential for utilizing renewable energy sources and starting materials in sustainable energy systems and chemical industries
The thermodynamically favored over-reduction of formaldehyde and its derivatives to the methanol level often prevails with chemical catalysts, making this important product platform difficult to exploit via direct reduction routes
Based on the obtained insights, we subsequently investigated the catalytic mechanism with density functional theory (DFT) methods analyzing the three catalytic cycles individually and combining them to the energy profile for the overall reaction sequence
Summary
The catalytic reduction of carbon dioxide (CO2) to valueadded products is essential for utilizing renewable energy sources and starting materials in sustainable energy systems and chemical industries. Our group recently reported that the cobalt triazine pincer complex 5 is a suitable catalyst for the selective hydrosilylation of CO2 to either formic acid, formaldehyde, or methanol derivatives using phenylsilane (PhSiH3) as a reducing agent (Figure 2). The data provide a coherent description of the energy landscape, explaining in detail how fine adjustments of the control parameters lead to a high selectivity for the three different CO2 reduction levels and suggesting general guidelines on how to optimize for the formaldehyde product platform. This fundamental knowledge provides tools for designing and developing new adaptive catalysts that allow selective access to the different C1reduction products
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