Cobalt-Catalyzed Hydrosilylation of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Level─How to Control the Catalytic Network?

Abstract

The selective hydrosilylation of carbon dioxide (CO 2 ) 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, CO 2 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 CO 2 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.

Hydrosilylative Reduction of CO2 to Formate and Methanol Using a Cobalt Porphyrin‐Based Porous Organic Polymer

“…When we investigated the mechanism of the reaction using 1 H NMR experiments, we did not observe the formation of a Co−H species upon the addition of Ph 2 SiH 2 (Figure S32, ESI). This intermediate is often considered to be the active hydride responsible for reducing CO 2 to formate for several organometallic catalytic systems [20,26–28] . Thus, we believe the CO 2 coordinates to Co‐BBT‐POP and undergoes a σ‐bond metathesis with Ph 2 SiH 2 to produce the diphenylsilane intermediate (Scheme S4, ESI).…”

“…In contrast to the homogeneous system, increasing the temperature of the reaction to 40 °C did not affect the efficiency of the reaction (Entry 13, Table 1). It should be noted that the TON values for the generation of potassium formate are higher than what has been reported for other Co‐based catalytic systems [26,27] . To test the recyclability of the system, Co‐BBT‐POP was collected by filtration and washed excessively with MeCN.…”

“…This intermediate is often considered to be the active hydride responsible for reducing CO 2 to formate for several organometallic catalytic systems. [20,[26][27][28] Thus, we believe the CO 2 coordinates to Co-BBT-POP and undergoes a σ-bond metathesis with Ph 2 SiH 2 to produce the diphenylsilane intermediate (Scheme S4, ESI). This intermediate can then be cleaved in the presence of KF to produce the potassium formate.…”

“…It should be noted that the TON values for the generation of potassium formate are higher than what has been reported for other Co-based catalytic systems. [26,27] To test the recyclability of the system, Co-BBT-POP was collected by filtration and washed excessively with MeCN. Afterwards, the catalyst was dried under vacuum and reused in the next reaction.…”

Rhodium(i) complexes derived from tris(isopropyl)-azaphosphatrane—controlling the metal–ligand interplay

“…As part of our research program aimed at developing direct molecular control over bond activation processes and catalysis, 22 we speculated that proazaphosphatrane ligands would be attractive molecular handles. The application of these ligands may provide an opportunity for deliberate regulation of the metal's immediate environment by varying the extent of transannulation throughout the catalytic process.…”