Towards a complete mechanistic understanding of the CO2 hydrogenation to methanol with the Ru-MACHO catalyst

One of the most efficient and studied homogeneous catalysts for CO2 hydrogenation to methanol is the Ru-MACHO-Ph complex. Nevertheless, the nature of the catalyst resting state (Ru-formate, Ru-carbamate, and Ru-CO⁺) and the contribution of different pathways for amide hydrogenation are remaining questions. In this work, GFN2-xTB, density functional theory (DFT) and DLPNO-CCSD(T) methods were used to explore the most plausible pathways proposed for the CO2 hydrogenation reaction assisted by dimethylamine (DMA). Microkinetic modeling was utilized to predict the methanol turnover number (TON), which agreed with experimental data, and to analyze the proposed mechanisms and catalyst resting states. The model indicated that the dominant dimethylformamide (DMF) hydrogenation pathway proceeds through a metal-ligand cooperative mechanism, followed by the hemiaminal protonolysis through an organic reaction (no Ru-catalyst required). In addition, Ru-formate (3) is the primary resting state, together with Ru-carbamate (8). Furthermore, we found that increasing the DMA concentration enhances the methanol TON, in agreement with previous experimental results.