Rational design of metal–ligands for the conversion of CH4 and CO2 to acetates: role of acids and Lewis acids.

The capture, sequestration and utilization of carbon dioxide have attracted global attention in the environmental field, science and industry. The concurrent conversion of CO₂ and CH₄ is a reasonable solution to reduce greenhouse gases, but it remains a challenging research topic. Herein, we reveal the catalytic activity of bifunctional half-sandwich complexes (metal = Ru, Rh, and Ir) in the direct conversion of methane and carbon dioxide to acetic acid (AA) using density functional theory. The use of Ru/Rh/Ir metals with ethanediphosphine (PP), N-tosylethylenediamine (NNTs), and ethylene glycol (OO) ligands shows superb performance in lowering the free energy (ΔG) barrier (ΔG^‡) for the conversion. To activate CH₄ and CO₂ molecules, we used extra additives to reduce the high energy barriers required by simple metal–ligand complexes. For CH₄ activation, additives of AA and trifluoroacetic acid (TFA) were used to assist the proton abstraction in the Ru–NNTs/OO complexes, while no acid additives were used for the PP ligands. In the case of the PP ligands, the C–H activation occurs through a "concerted oxidative addition–reductive elimination" (OA–RE) process, whereas in the NNTs/OO ligands, C–H activation occurs through "cyclometallation deprotonation" (CMD) irrespective of the presence of acid additives. The reduction in ΔG^‡ for CH₄ activation in Ru–NNTs/OO is 5–10 kcal mol⁻¹ using acid additives. In contrast, in Ru–PP, this ΔG^‡ barrier is 20 kcal mol⁻¹ without additives. For CO₂ activation, AlCl₃ (Lewis acid) was used to insert CO₂ into the metal site of the Ru–NNTs/OO and Ru/Rh/Ir–PP complexes. The reduction in ΔG^‡ using AlCl₃ for CO₂ activation in Ru–NNTs·AA/OO·2TFA and Ru/Rh/Ir–PP is 15–30 kcal mol⁻¹. Then, the overall activation barrier is reduced to 20–27 kcal mol⁻¹. [ABSTRACT FROM AUTHOR]