Oxalate and Acrylate from CO2

Carbon dioxide is a greenhouse gas but also has the potential to become an important carbon source for value-added products. Processes that, for example, reductively convert CO₂ to products such as formic acid or methanol are useful. The direct formation of molecules containing C–C bonds, such as ethylene, acrylate, or oxalate, from CO₂ is especially promising because these molecules are important industrial raw materials. However, efficiently converting CO₂ to these compounds is more challenging than producing C1-compounds.

Wesley H. Bernskoetter, University of Missouri, Columbia, USA, and colleagues have developed an iron-complex-mediated reductive coupling reaction that can produce oxalate or acrylate from CO₂ (pictured). Iron(0)-complexes carrying two 1,2-bis(diethylphosphino)ethane (depe) ligands and either ethylene or CO₂ as an additional ligand L were used to mediate the formation of acrylate and oxalate, respectively. The Fe(0)-complex was synthesized via the reduction of the corresponding Fe(II)complex by potassium graphite under nitrogen, which acts as a labile ligand for the Fe(0)-complex.

Replacing the nitrogen ligand by ethylene in situ and subsequent treatment with CO₂ and addition of a base led to the formation of acrylate in stoichiometric amounts via an iron-lacton intermediate. Attempts to use the complex catalytically were not successful, however. To produce oxalate, the Fe(depe)₂ complex bearing a CO₂-ligand was used and treated with KC₈ in tetrahydrofuran (THF) under 1.5 atm of CO₂ at 45 °C. This reaction gave an excellent oxalate yield of 98 % when stoichiometric amounts of the iron complex were used.

Overall, Fe(depe)₂L successfully mediates the formation of acrylate from CO₂ and ethylene, as well as the coupling of two CO₂ molecules to form oxalate. The reactions cannot be run with catalytic amounts of iron, however. This is presumably due to the complex’s stable 18-electron configuration, which disfavors β-hydride elimination in case of the acrylate formation. For the oxalate formation, there is a competing CO₂ disproportionation. Therefore, further studies on more efficient iron complexes are necessary.