PNNL

The Science                                

Many industrially relevant catalytic reactions are run at large scales with solid catalysts. While these processes may be efficient, identifying precise reaction mechanisms can be challenging. Researchers used a gas–solid reaction between crystals of a copper (Cu)-containing molecule and carbon dioxide (CO₂) to study CO₂ insertion into Cu-H-Cu bonds. They observed two SC-SC insertions of CO₂ into the Cu-H-Cu bond, which demonstrated unexpectedly high reactivity. When the solid crystals were dissolved in a solvent, the complexes cleanly break apart. This work identified a new route to copper-based formate complexes that may be relevant in CO₂ catalysis.

The Impact

Converting CO₂ to fuels is an important part of effectively closing the carbon cycle. In this challenging process, understanding the different potential mechanisms enables researchers to create optimized catalysts. This work uses molecular complexes to study potential mechanisms at play in solid heterogeneous catalysts.

Summary

Insertion of CO₂ into metal hydride bonds is a pivotal step in the catalytic reduction of CO₂ to fuels. Studies of molecular complexes can provide a useful and highly detailed understanding of reaction mechanisms. This work studied the reaction of CO₂ and a solid copper (Cu)-based organometallic complex in a single-crystal to single-crystal (SC-SC) transformation, providing a crucial link between homogeneous and heterogeneous catalysis. When exposed to CO₂ gas for 15 minutes, insertion of CO₂ into a bridging Cu-H-Cu bond converts SCs of [(IPr*OMe)CuH]₂ to SCs of a dicopper formate hydride [(IPr*OMe)Cu]₂(1,3-O₂CH)(H). Remarkably, after three hours of exposure to CO₂, a second SC-SC reaction occurs through insertion into the remaining Cu-H-Cu bond. The bridging formate complexes provide insights into proposed intermediates in reduction of CO₂. Upon dissolution in a solvent, both of the bridging formate crystals rupture cleanly into mononuclear copper complexes. The gas-solid reactions provide insight into a new reaction pathway for these complexes. In general, insertion of any substrate into a bridging hydride is disfavored compared to terminal metal hydride bonds, so these results illustrate surprisingly high reactivity of the bridging hydride bonds.

PNNL Contact

Ba Tran, Pacific Northwest National Laboratory, ba.tran@pnnl.gov

Funding

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, Catalysis Science Program, FWP 47319.