Mechanistic Insight into Acceptorless Dehydrogenation of Methanol to Syngas Catalyzed by MACHO-Type Ruthenium and...

The acceptorless dehydrogenation of methanol to produce carbon monoxide (CO) and dihydrogen (H₂) mediated by MACHO-type 1-Ru and 1-Mn complexes was theoretically investigated via density functional theory calculations. The 1-Ru-catalyzed process involves the formation of active species 4-Ru through a methanol-bridged H₂ release pathway. Methanol dehydrogenation by 4-Ru yields formaldehyde and 1-Ru, followed by H₂ release to regenerate 4-Ru (rate-determining step, ΔG^‡ = 32.5 kcal/mol). Formaldehyde further reacts with methanol via nucleophilic attack of the MeO^- ligand in the Ru complex (ΔG^‡ = 9.6 kcal/mol), which is more favorable than the traditional methanol-to-formaldehyde nucleophilic attack (ΔG^‡ = 33.8 kcal/mol) due to the higher nucleophilicity of MeO^-. CO is ultimately produced through the methyl formate decarbonylation reaction. Accelerated H₂ release in the early reaction stage compared to CO results from the initial methanol dehydrogenation and condensation of formaldehyde with methanol. In contrast, CO generation occurs later via methyl formate decarbonylation. The 1-Mn-catalyzed reaction has reduced efficiency compared to 1-Ru for the higher Gibbs energy barrier (ΔG^‡ = 34.1 kcal/mol) of the rate-determining step. Excess NaOBu promotes the reaction of CO and methanol, forming methyl formate, significantly reducing the CO/H₂ ratio as the catalyst amount decreases. These findings deepen our understanding of the methanol-to-syngas transformation and can drive progress in this field.