Combined Computational and Experimental Investigation on the Mechanism of CO₂ Hydrogenation to Methanol with Mn-PNP-Pincer Catalysts

Abstract

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.

Activated Mn‐MACHO Complexes Form Stable CO2 Adducts

“…The derived molecular structure of 1‐CO 2 (Figure 3) displays the four‐membered {Mn−N−C−O} metallacyclic unit typical for pincer‐type complexes coordinating CO 2 via MLC pathways. This observation is in line with our previous DFT calculations, which predict the formation of this adduct upon reaction of the amido complex 1 with CO 2 [4a] …”

“…On the other hand, catalytic transformations involving MLC‐type reaction pathways at Mn‐MACHO systems might therefore be substantially slowed down in the presence of CO 2 by formation of stable Mn−(CO 2 ) adducts. This point substantiates an additional rationale to the experimentally observed negative CO 2 reaction order in CO 2 ‐to‐methanol hydrogenation catalysed by 1 [4a] . Along the same line, our findings also corroborate the reported lower reactivity of 1 in hydrogenation of CO 2 to formate salts compared to the N ‐methyl‐substituted analogue [ Mn i PrMe −H ], [4h] as the later cannot form such stable, possibly inhibiting CO 2 adduct.…”

“…Despite extensive investigations of Mn‐MACHO‐type complexes including [ Mn i PrH −Br ] (Figure 1c) as catalysts for CO 2 conversion, [4a,g–i,5] the formation of such adducts has not been reported so far for that class of compounds. Only the corresponding coordination of isoelectronic isocyanates to the M=N unit has been recently issued [11] .…”

Hydroboration Reduction of CO2 Catalyzed by a Doubly Reduced Arylborane: DFT Insight into Double Active-Site and CO2 Self-Promoting Single Active-Site Mechanisms and Counterion Effects

“…Similar to the metal–ligand cooperation (MLC) mechanism for the CO/CN bond reduction catalyzed by transition-metal complexes, − ,− here, the double active-site mechanism of the CO 2 hydroboration catalyzed by A0 through three catalytic subcycles is proposed in Figure . The possible intermediates and transition states have been investigated, and the predicted relative energies of the stationary points along the reaction pathway are relative to A0 .…”

Cyclopentadienone Diphosphine Ruthenium Complex: A Designed Catalyst for the Hydrogenation of Carbon Dioxide to Methanol

“…This iron-based catalyst with a tripodal scorpionate ligand is very promising for potential industrial applications with a carbon efficiency of 86% and an atom economy of 62%. In 2021, Leitner et al reported a direct HCM process catalyzed by the Mn-PNP pincer complex with a TON of 160 ( p (CO 2 ) = 5 bar, p (H 2 ) = 160 bar, T = 150 °C). , Although progress has been achieved in the HCM, most of the reported catalytic systems either contain low TONs or require rigid reaction conditions. Therefore, the design of efficient homogeneous catalysts with new strategies is highly attractive.…”