Dedicated to Professor Christian Bruneau on the occasion of his 70 th birthday.The hydrogenation of CO 2 to methanol was achieved using a catalytic system comprising metal complexes of the form [Mn(CO) 2 [N(C 2 H 4 PR 2 )] (R = i Pr/Ph, [HN(C 2 H 4 PPh 2 ) 2 ] = MACHOÀ Ph) together with Lewis acid additives. Mechanistic studies suggest initial CO 2 insertion into a MnÀ H bond leads to a formate complex as resting state. By systematically balancing the interaction between the acidic additive and the catalyst, the formate ligand could be removed through esterification to [a
A theoretical and experimental mechanistic study is presented for the homogeneously catalyzed CO 2 hydrogenation to methanol, using an Mn-PNP-Pincer catalyst in the presence of a Lewis acid cocatalyst and alcohol as a solvent. Quantum chemical computations at the density functional theory and DLPNO-CCSD(T) level of theory suggest the presence of a formate resting state as the most stable intermediate. The concerted activation of dihydrogen via a proton shuttle mechanism and decomposition of a hemiacetal intermediate is computed to define the turnoverdetermining transition state. The resulting energy span is calculated as 34.5 kcal mol −1 at the DLPNO-CCSD(T) level of theory. An Eyring plot reveals the experimental barrier of the reaction at 31.4 kcal mol −1 under catalytic turnover conditions, showing a good agreement with a slight overestimation of the computational model. Concentration−time profiles of the involved species also locate experimentally the rate-determining states (RDSs) of the reaction in the hydrogenation of a formate ester intermediate to methanol. The measured kinetic isotope effects for the use of H 2 /D 2 and EtOH/D are in agreement with hydrogen splitting as the RDS, giving further support to the computed mechanism. These insights provide guidance and reference for future improvement of hydrogenation catalysts based on abundant 3d metals for the CO 2 -based production of methanol.
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