A theoretical study on the hydrogenation of CO2 to methanol catalyzed by ruthenium pincer complexes

Herein, a density functional theory (DFT) study was performed to investigate thoroughly the cascade reaction mechanism for the hydrogenation of carbon dioxide to methanol catalyzed by ruthenium pincer complex [RuH₂(Me₂PCH₂SiMe₂)₂NH(CO)]. Three catalytic stages involving the hydrogenation of carbon dioxide (stage I), formic acid (stage II) and formaldehyde (stage III) were studied. The calculated results show that the dominant H₂ activation strategy in the hydrogenation of CO₂ to methanol may not be the methanol-assisted H₂ activation, but the formate-assisted H₂ activation. In this cascade reaction, all energy spans of stage I, II and III are 20.2 kcal mol^-1 of the formate-assisted H₂ activation. This implies that it could occur under mild conditions. Meanwhile, the catalyst is proposed to be efficient for the transfer hydrogenation using isopropanol as the hydrogen resource, and the ruthenium pincer complexes [RuH₂(Me₂PCH₂SiMe₂)₂NH(CO)], [RuH₂(Ph₂PCH₂SiMe₂)₂NH(CO)] and [RuH₂(Me₂PCH₂SiMe₂)₂NH(CO)] exhibit similar catalytic activities for the hydrogenation of CO₂ to methanol.

Hydroboration of CO2 to Methyl Boronate Catalyzed by a Manganese Pincer Complex: Insights into the Reaction Mechanism and Ligand Effect

The conversion of carbon dioxide to fuels, polymers, and chemicals is an attractive strategy for the synthesis of high-value-added products and energy-storage materials. Herein, the density functional theory method was employed to investigate the reaction mechanism of CO₂ hydroboration catalyzed by manganese pincer complex, [Mn(Ph₂PCH₂SiMe₂)₂NH(CO)₂Br]. The carbonyl association and carbonyl dissociation mechanisms were investigated, and the calculated results showed that the carbonyl association mechanism is more favorable with an energetic span of 27.0 kcal/mol. Meanwhile, the solvent effect of the reaction was explored, indicating that the solvents could reduce the catalytic activity of the catalyst, which was consistent with the experimental results. In addition, the X ligand effect (X = CO, Br, H, PH₃) on the catalytic activity of the manganese complex was explored, indicating that the anionic complexes [Mn^I - Br]^- and [Mn^I - H]^- have higher catalytic activity. This may not only shed light on the fixation and conversion of CO₂ catalyzed by earth-abundant transition-metal complexes but also provide theoretical insights to design new transition-metal catalysts.

A phosphine-free Mn(I)-NNS catalyst for asymmetric transfer hydrogenation of acetophenone: a theoretical prediction

The density functional theory (DFT) method was employed to investigate the reaction mechanism of the hydrogen activation and asymmetric transfer hydrogenation (ATH) of acetophenone catalyzed by a well-defined phosphine-free Mn(I)-NNS complex. The calculation results indicate that the Mn-NNS complex has potential high catalytic hydrogenation activity. Meanwhile, the hydrogen transfer step of this reaction is proposed to be a concerted but asynchronous process, and the hydride transfer precedes proton transfer. This work also pointed out that the stereoselectivity of ATH catalyzed by the Mn(I)-NNS complex mainly originates from the noncovalent interaction between the substrate and the catalyst. Additionally, the catalytic activities of Mn-NNS complexes with different X ligands (X = CO, Cl, H, OMe, NCMe, CCMe, and CHCHMe) were compared, and the calculated total reaction energy barriers were all viable, which indicates that these Mn-NNS complexes show higher CO bond hydrogenation activity under mild conditions. This theoretical study predicts that the reactions catalyzed by complexes with H and NCMe ligands exhibit high stereoselectivity with enantiomeric excess (ee) values of 97% and 93%, respectively.

Hydrogenation of CO2 to methanol catalyzed by a manganese pincer complex: insights into the mechanism and solvent effect

Herein, density functional theory (DFT) calculations were employed to explore the reaction mechanism of three cascade cycles for the hydrogenation of carbon dioxide to methanol (CO2 + 3H2 → CH3OH + H2O) catalyzed by a manganese pincer complex [Mn(Ph2PCH2SiMe2)2N(CO)2]. The three cascade cycles involve: the hydrogenation of CO2 to formic acid, the hydrogenation of formic acid to methanediol and the decomposition of methanediol to formaldehyde and water, and the hydrogenation of formaldehyde to methanol. The calculated results demonstrate that hydrogen activation is the rate-determining step of each catalytic cycle under solvent-free conditions, and the energy span of the whole reaction is 27.1 kcal mol-1. Furthermore, the solvent was found to be of importance in this reaction. In three different solvents, the rate-determining steps of this reaction are all the hydrogen transfer step of the formic acid hydrogenation stage, and the corresponding energy spans in water, toluene and THF solvents are 21.3, 20.8 and 20.4 kcal mol-1, respectively. Such a low energy span implies that this manganese complex could be a promising catalyst for the efficient conversion of CO2 and H2 to methanol at temperatures below 100-150 °C.

Using Bases as Initiators to Isomerize Allylic Alcohols: Insights from Density Functional Theory Studies

Allylic alcohols, as common and readily available building blocks, could be converted into many widely used carbonyl compounds through isomerization reactions. However, these processes often involve expensive transition metal (TM) complexes as the catalyst. What is the bottleneck in the mechanism when no TM is used? In this study, density functional theory (DFT) was employed to explore the mechanistic patterns of allylic alcohols catalyzed using bases, such as KOH, NaOH, LiOH, tBuOK, tBuONa, tBuOLi, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine, and 1,8-diazabicyclo[5.4.0]undec-7-ene. Our results show that bases containing metal cations follow the metal cation-assisted (MCA) mechanism, whereas organic bases without metal cations follow the ion pair-assisted (IPA) mechanism. The catalytic efficiency of bases containing metal cations is higher than that of bases without metal cations, indicating that metal cations play an important role in the reaction. Additionally, the modulation of substituents R₁ and R₂ in the substrate reveals that electron-withdrawing groups are favorable for C-H bond cleavage, and electron-donating groups are favorable for hydrogen transfer. To better understand these patterns, we applied the DFT and information-theoretic approach (ITA) to examine the impact of bases and substrate substituents on the reactivity of allylic alcohol isomerization. This work should provide a much-needed theoretical guidance to design better non-TM catalysts for the isomerization of allylic alcohols and their derivatives.

Asymmetric Induction with a Chiral Amine Catalyzed by a Ru-PNP Pincer Complex: Insight from Theoretical Investigation

In this paper, the mechanism of asymmetric amination of a racemic alcohol with Ellman's sulfinamide and the origin of diastereoselectivity catalyzed by a Ru-PNP pincer complex were studied using density functional theory (DFT). The mechanism involves dehydrogenation of the racemic alcohol, C-N coupling, and hydrogen transfer from the catalyst to the in situ formed imine. The calculated results indicate that both the alcohol dehydrogenation and imine hydrogenation are stepwise. The hydride transfer from a Ru hydride complex to the imine is shown to be the chirality-determining step in the whole catalytic cycle. It was found that the diastereoselectivity mainly stems from the hydrogen bonding interactions between the oxygen atom of the sulfinyl moiety and the hydrogen atom of the NH group of the ligand.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

The enantioselectivity in asymmetric ketone hydrogenation catalyzed by RuH2(diphosphine)(diamine) complexes: insights from a 3D-QSSR and DFT study

The 3D-QSSR method was carried out to investigate the enantioselectivity of the asymmetric ketone hydrogenation (AKH) catalyzed by RuH₂(diphosphine)(diamine) complexes integrating with DFT method, which could provide a way to design homogeneous transition-metal catalysts.

The N-H functional group in organometallic catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst-substrate, ligand-ligand, ligand-catalyst, and catalyst-catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.

Concerted or stepwise hydrogen transfer in the transfer hydrogenation of acetophenone catalyzed by ruthenium-acetamido complex: a theoretical mechanistic investigation

In this paper, the mechanism of transfer hydrogenation of acetophenone catalyzed by ruthenium-acetamido complex was studied using density function theory (DFT) method. The catalytic cycle of transfer hydrogenation consists of hydrogen transfer (HT) step and dehydrogenation (DH) step of isopropanol (IPA). Inner sphere mechanism (paths 1 and 7) and outer sphere mechanism (paths 2-6) in HT step are fully investigated. Calculated results indicate that DH step of IPA (from (i)1 to (i)2) is the rate-determining step in the whole catalytic cycle, which has a potential energy barrier of 16.2 kcal/mol. On the other hand, the maximum potential energy barriers of paths 1-7 in the HT step are 5.9, 12.7, 24.4, 16.8, 23.7, 7.2, and 6.1 kcal/mol, respectively. The inner sphere pathways (paths 1 and 7) are favorable hydrogen transfer modes compared with outer sphere pathways, and the proton transferred to the oxygen atom of acetophenone comes from the hydroxyl group but not from amino group of acetamido ligand. Those theoretical results are in agreement with experimental report. However, in view of this DFT study in the inner sphere mechanism of HT step, hydride transfer and proton transfer are concerted and asynchronous hydrogen transfer but not a stepwise one, and hydride transfer precedes proton transfer in this case.