Mechanistic Insight into Acceptorless Dehydrogenation of Methanol to Syngas Catalyzed by MACHO-Type Ruthenium and Manganese Complexes: A DFT Study

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 H-2 release pathway. Methanol dehydrogenation by 4-Ru yields formaldehyde and 1Ru, followed by H-2 release to regenerate 4-Ru (rate-determining step, Delta G double dagger = 32.5 kcal/mol). Formaldehyde further reacts with methanol via nucleophilic attack of the MeO- ligand in the Ru complex (Delta G double dagger = 9.6 kcal/mol), which is more favorable than the traditional methanol-to formaldehyde nucleophilic attack (Delta G double dagger = 33.8 kcal/mol) due to the higher nucleophilicity of MeO-. CO is ultimately produced through the methyl formate decarbonylation reaction. Accelerated H-2 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 (Delta G double dagger = 34.1 kcal/mol) of the rate-determining step. Excess (NaOBu)-Bu-t promotes the reaction of CO and methanol, forming methyl formate, significantly reducing the CO/H-2 ratio as the catalyst amount decreases. These findings deepen our understanding of the methanol-to-syngas transformation and can drive progress in this field.

Automated summary

The acceptorless dehydrogenation of methanol to produce carbon monoxide (CO) and dihydrogen (H2) mediated by MACHO-type 1-Ru and 1-Mn complexes was investigated. The 1-Ru-catalyzed process involves the formation of active species 4-Ru through a methanol-bridged H-2 release pathway. Methanol dehydrogenation by 4-Ru yields formaldehyde and 1Ru, followed by H-2 release to regenerate 4-Ru. The 1-Mn-catalyzed reaction has reduced efficiency compared to 1-Ru due to a higher Gibbs energy barrier in the rate-determining step.