Hydrogenation of CO2 to Methanol Catalyzed by Cp*Co Complexes: Mechanistic Insights and Ligand Design

Mechanism of CO2 Reduction to Methanol with H2 on an Iron(II)-scorpionate Catalyst

CO2 utilization is an important process in the chemical industry with great environmental power. In this work we show how CO2 and H2 can be reacted to form methanol on an iron(II) center and highlight the bottlenecks for the reaction and what structural features of the catalyst are essential for efficient turnover. The calculations predict the reactions to proceed through three successive reaction cycles that start with heterolytic cleavage of H2 followed by sequential hydride and proton transfer processes. The H2 splitting process is an endergonic process and hence high pressures will be needed to overcome this step and trigger the hydrogenation reaction. Moreover, H2 cleavage into a hydride and proton requires a metal to bind hydride and a nearby source to bind the proton, such as an amide or pyrazolyl group, which the scorpionate ligand used here facilitates. As such the computations highlight the non-innocence of the ligand scaffold through proton shuttle from H2 to substrate as an important step in the reaction mechanism.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Formaldehyde (HCHO) is a crucial C₁ building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO₂ ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO₂ -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO₂ to HCHO are discussed.

Ligand-assisted Hydride Transfer: A Pivotal Step for CO2 Hydroboration Catalyzed by a Mononuclear Mn(I) PNP Complex

A mononuclear Mn(I) pincer complex [Mn(Ph₂ PCH₂ SiMe₂ )₂ NH(CO)₂ Br] was disclosed to catalyze the pinacolborane (HBpin)-based CO₂ hydroboration reaction. Density functional calculations were conducted to reveal the reaction mechanism. The calculations showed that the reaction mechanism could be divided into four stages: (1) the addition of HBpin to the unsaturated catalyst C1; (2) the reduction of CO₂ to HCOOBpin; (3) the reduction of HCOOBpin to HCHO; (4) the reduction of HCHO to CH₃ OBpin. The activation of HBpin is the ligand-assisted addition of HBpin to the unsaturated Mn(I)-N complex C1 generated by the elimination of HBr from the Mn(I) pincer catalyst. The sequential substrate reductions share a common mechanism, and every hydroboration commences with the nucleophilic attack of the Mn(I)-H to the electron-deficient carbon centers. The hydride transfer from Mn(I) to HCOOBpin was found to be the rate-limiting step for the whole catalytic reaction, with a total barrier of 27.0 kcal/mol, which fits well with the experimental observations at 90 °C. The reactivity trend of CO₂ , HCOOBpin, HCHO, and CH₃ OBpin was analyzed through both thermodynamic and kinetic analysis, in the following order, namely HCHO>CO₂ >HCOOBpin≫CH₃ OBpin. Importantly, the very high barrier for the reduction of CH₃ OBpin to form CH₄ reconciles with the fact that methane was not observed in this catalytic reaction.

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.

Unraveling the catalytically preferential pathway between the direct and indirect hydrogenation of CO2 to CH3OH using N-heterocyclic carbene-based Mn(i) catalysts: a theoretical approach

The mechanistic investigation of direct vs. indirect CO₂ hydrogenation to methanol using single molecular NHC-based Mn(i) complexes.

Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions

This review summarizes the concepts, mechanisms, drawbacks and challenges of the state-of-the-art catalysis for CO₂ to MeOH under mild conditions. Thoughtful guidelines and principles for future research are presented and discussed.

CO2 Reduction to Methanol in the Liquid Phase: A Review

Excessive carbon dioxide (CO₂ ) emissions have been subject to extensive attention globally, since an enhanced greenhouse effect (global warming) owing to a high CO₂ concentration in the atmosphere could lead to severe climate change. The use of solar energy and other renewable energy to produce low-cost hydrogen, which is used to reduce CO₂ to produce bulk chemicals such as methanol, is a sustainable strategy for reducing carbon dioxide emissions and carbon resources. CO₂ conversion into methanol is exothermic, so that low temperature and high pressure are favorable for methanol formation. CO₂ is usually captured and recovered in the liquid phase. Herein, the emerging technologies for the hydrogenation of CO₂ to methanol in the condensed phase are reviewed. The development of homogeneous and heterogeneous catalysts for this important hydrogenation reaction is summarized. Finally, mechanistic insight on CO₂ 's conversion into methanol over different catalysts is discussed by taking the available reaction pathways into account.

Bioinspired Design and Computational Prediction of SCS Nickel Pincer Complexes for Hydrogenation of Carbon Dioxide

Inspired by the structures of the active site of lactate racemase and H2 activation mechanism of mono-iron hydrogenase, we proposed a series of sulphur–carbon–sulphur (SCS) nickel complexes and computationally predicted their potentials for catalytic hydrogenation of CO2. Density functional theory calculations reveal a metal–ligand cooperated mechanism with the participation of a sulfur atom in the SCS pincer ligand as a proton receiver for the heterolytic cleavage of H2. For all newly proposed complexes containing functional groups with different electron-donating and withdrawing abilities in the SCS ligand, the predicted free energy barriers for the hydrogenation of CO2 to formic acid are in a range of 22.2–25.5 kcal/mol in water. Such a small difference in energy barriers indicates limited contributions of those functional groups to the charge density of the metal center. We further explored the catalytic mechanism of the simplest model complex for hydrogenation of formic acid to formaldehyde and obtained a total free energy barrier of 34.6 kcal/mol for the hydrogenation of CO2 to methanol.

Phosphine-free ruthenium NCN-ligand complexes and their use in catalytic CO2 hydrogenation

This work investigates the hydrogenation of carbon dioxide to formate catalysed by the phosphine-free Ru complexes Ru(OtBu)(κ³-NCN)(^tBubpy) and RuH(OtBu)(κ²-NCN)(^tBubpy) (OtBu = tert-butoxide, κ²-NCN = 1,3-di(2-methylpyridyl)-4,5-diphenyl-1H-imidazol-2-ylidene, where one pyridyl moiety is not coordinated to Ru, ^tBubpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). A catalytic cycle is proposed for this reaction, supported by computational studies and the characterization of the hydride and the formate intermediates proposed to be involved. Modest catalytic turnovers are demonstrated at relatively low pressures and temperatures. The proposed rate determining step is heterolytic H₂ splitting to regenerate the Ru-H complex, which has an estimated hydricity of approx. 27 kcal mol^-1. The κ²-NCN ligand in the hydride complex undergoes a variety of dynamic processes as detected by EXSY spectroscopy including a pyridyl "roll-over" carbon-hydrogen - ruthenium hydride exchange, possibly occuring via a Perutz-Sabo-Etienne CAM mechanism.