Iron catalyzed CO2 hydrogenation to formate enhanced by Lewis acid co-catalysts

Mechanism of iron complexes catalyzed in the N-formylation of amines with CO2 and H2: the superior performance of N-H ligand methylated complexes

CO2 hydrogenation into value-added chemicals not only offer an economically beneficial outlet but also help reduce the emission of greenhouse gases. Herein, the density functional theory (DFT) studies have been carried out on CO2 hydrogenation reaction for formamide production catalyzed by two different N-H ligand types of PNP iron catalysts. The results suggest that the whole mechanistic pathway has three parts: (i) precatalyst activation, (ii) hydrogenation of CO2 to generate formic acid (HCOOH), and (iii) amine thermal condensation to formamide with HCOOH. The lower turnover number (TON) of a bifunctional catalyst system in hydrogenating CO2 may attribute to the facile side-reaction between CO2 and bifunctional catalyst, which inhibits the generation of active species. Regarding the bifunctional catalyst system addressed in this work, we proposed a ligand participated mechanism due to the low pKa of the ligand N-H functional in the associated stage in the catalytic cycle. Remarkably, catalysts without the N-H ligand exhibit the significant transfer hydrogenation through the metal centered mechanism. Due to the excellent catalytic nature of the N-H ligand methylated catalyst, the N-H bond was not necessary for stabilizing the intermediate. Therefore, we confirmed that N-H ligand methylated catalysts allow for an efficient CO2 hydrogenation reaction compared to the bifunctional catalysts. Furthermore, the influence of Lewis acid and strong base on catalytic N-formylation were considered. Both significantly impact the catalytic performance. Moreover, the catalytic activity of PNMeP-based Mn, Fe and Ru complexes for CO2 hydrogenation to formamides was explored as well. The energetic span of Fe and Mn catalysts are much closer to the precious metal Ru, which indicates that such non-precious metal catalysts have potentially valuable applications.

Computational Study of Triphosphine-Ligated Cu(I) Catalysts for Hydrogenation of CO2 to Formate

The catalyzed hydrogenation of CO₂ to formate via a triphosphine-ligated Cu(I) was studied computationally at the density functional theory level in the presence of a self-consistent reaction field. Of the four functionals benchmarked, M06 was generally in the best agreement with the available experimentally estimated values. Two bases, DBU and TBD, were studied in the context of two proposed mechanisms in the MeCN solvent. Activation of H₂ was explored by using LCu(DBU)⁺ to form LCuH. Dissociation of a ligand arm results in higher barriers to form the key hydride complex, LCuH. The preferred mechanism passes through a transition state, where the H₂ has one H atom interacting with the copper center and the other H atom interacting with the N atom of the base, similar to H₂ insertion into a frustrated Lewis pair. There is no significant difference between the choice of a base, DBU or TBD, with respect to the proposed mechanisms. We propose that the experimentally observed differences between DBU and TBD reactivities for this mechanism are due to off-pathway changes.

Carbon Dioxide Hydrogenation to Formate Catalyzed by a Bench-Stable, Non-Pincer-Type Mn(I) Alkylcarbonyl Complex

The catalytic reduction of carbon dioxide is a process of growing interest for the use of this simple and abundant molecule as a renewable building block in C1-chemical synthesis and for hydrogen storage. The well-defined, bench-stable alkylcarbonyl Mn(I) bis(phosphine) complex fac-[Mn(CH₂CH₂CH₃)(dippe)(CO)₃] [dippe = 1,2-bis(diisopropylphosphino)ethane] was tested as an efficient and selective non-precious-metal precatalyst for the hydrogenation of CO₂ to formate under mild conditions (75 bar total pressure, 80 °C), in the presence of a Lewis acid co-catalyst (LiOTf) and a base (DBU). Mechanistic insight into the catalytic reaction is provided by means of density functional theory (DFT) calculations.

Strategies for Integrated Capture and Conversion of CO2 from Dilute Flue Gases and the Atmosphere

The integrated capture and conversion of CO₂ has the potential to make valorization of the greenhouse gas more economically competitive, by eliminating energy-intensive regeneration processes. However, integration is hindered by the extremely low concentrations of CO₂ present in the atmosphere (0.04 vol.%), and the presence of acidic gas contaminants, such as SO_x and NO_x , in flue gas streams. This Review summarizes the latest technological progress in the integrated capture and conversion of CO₂ from dilute flue gases and atmospheric air. In particular, the Review analyzes the correlation between material properties and their capture and conversion efficiency through hydrogenation, cycloaddition, and solar thermal-mediated electrochemical processes, with a focus on the types and quantities of product generated, in addition to their energy requirements. Prospects for commercialization are also highlighted and suggestions are made for future research.

CO2 Hydrogenation Catalyzed by a Ruthenium Protic N-Heterocyclic Carbene Complex

We describe the hydrogenation of CO₂ to formate catalyzed by a Ru(II) bis(protic N-heterocyclic carbene, p-NHC) phosphine complex [Ru(bpy)(MeCN)(P^Ph(p-NHC)₂)](PF₆)₂ (1). Under catalytic conditions (20 μmol catalyst, 20 bar CO₂, 60 bar H₂, 5 mL THF, 140 °C, 16 h), the activity of 1 is limited only by the amount of K₃PO₄ present in the reaction, yielding a nearly 1:1 ratio of turnover number (TON) to equivalents of K₃PO₄ (relative to 1), with the highest TON = 8040. Additionally, analysis of the reaction solution post-run reveals the catalyst intact with no free ligand observed. Stoichiometric studies, including examination of unique carbamate and hydride complexes as relevant intermediates, were carried out to probe the operative mechanism and understand the importance of metal-ligand cooperativity in this system.

Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions

The five new copper(I) complexes [Cu₂(μ-Cl)₂(κ¹-PCP^t-Bu)] (1), [Cu₂(μ-Br)₂(κ¹-PCP^t-Bu)] (2), [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3), [Cu₂(μ-CN)₂(κ¹-PCP^t-Bu)] (4), and [Cu₄(μ₃-SCN)₄(κ¹-PCP^t-Bu)₂]·CH₂Cl₂ (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) Å, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCP^t-Bu ligand in a κ¹-manner and two bridging (pseudo)halido ligands in a μ₂-bonding mode to generate a nonplanar Cu₂(μ-X)₂ framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCP^t-Bu ligands in a κ¹-manner and the four bridging thiocyanate groups in a μ₃-manner. These complexes were used as catalysts for the hydrogenation of CO₂ to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO₂ hydrogenation.

Formate-Bicarbonate Cycle as a Vehicle for Hydrogen and Energy Storage

In recent years, hydrogen has been considered a promising energy carrier for a sustainable energy economy in the future. An easy solution for the safer storage of hydrogen is challenging and efficient methods are still being explored in this direction. Despite having some progress in this area, no cost-effective and easily applicable solutions that fulfill the requirements of industry are yet to be claimed. Currently, the storage of hydrogen is largely limited to high-pressure compression and liquefaction or in the form of metal hydrides. Formic acid is a good source of hydrogen that also generates CO₂ along with hydrogen on decomposition. Moreover, the hydrogenation of CO₂ is thermodynamically unfavorable and requires high energy input. Alkali metal formates are alternative mild and noncorrosive sources of hydrogen. On decomposition, these metal formates release hydrogen and generate bicarbonates. The generated bicarbonates can be catalytically charged back to alkali formates under optimized hydrogen pressure. Hence, the formate-bicarbonate-based systems being carbon neutral at ambient condition has certain advantages over formic acid. The formate-bicarbonate cycle can be considered as a vehicle for hydrogen and energy storage. The whole process is carbon-neutral, reversible, and sustainable. This Review emphasizes the various catalytic systems employed for reversible formate-bicarbonate conversion. Moreover, a mechanistic investigation, the effect of temperature, pH, kinetics of reversible formate-bicarbonate conversion, and new insights in the field are also discussed in detail.

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts.

Dehydrogenation of Formic Acid to CO2 and H2 by Manganese(I)–Complex: Theoretical Insights for Green and Sustainable Route

In this work, a detailed computational study on a recently synthetized Mn(I)-dependent complex [(tBuPNNOP)Mn(CO)2]+ is reported. This species promotes the dehydrogenation of formic acid to carbon dioxide and hydrogen. The here proposed catalytic cycle proceeds through the formation of stabilized adduct between [(tBuPNNOPtBu)Mn(CO)2]+ and formate and the progressive release of CO2 and H2, mediated by the presence of trimethylamine. In order to evaluate the influence of the environment on the catalytic activity, different solvents have been taken into account. The computed barriers and the geometrical parameters account well for the available experimental data, confirming the robustness of the complex and reproducing its good catalytic performance. Outcomes from the present investigation can stimulate further experimental works in the design of new more efficient catalysts devoted to H2 production.

Efficient carbon dioxide hydrogenation to formic acid with buffering ionic liquids

The efficient transformation of CO₂ into chemicals and fuels is a key challenge for the decarbonisation of the synthetic production chain. Formic acid (FA) represents the first product of CO₂ hydrogenation and can be a precursor of higher added value products or employed as a hydrogen storage vector. Bases are typically required to overcome thermodynamic barriers in the synthesis of FA, generating waste and requiring post-processing of the formate salts. The employment of buffers can overcome these limitations, but their catalytic performance has so far been modest. Here, we present a methodology utilising IL as buffers to catalytically transform CO₂ into FA with very high efficiency and comparable performance to the base-assisted systems. The combination of multifunctional basic ionic liquids and catalyst design enables the synthesis of FA with very high catalytic efficiency in TONs of >8*10⁵ and TOFs > 2.1*10⁴ h⁻¹.

Basic ionic liquids provide a buffering effect that enables the efficient synthesis of free formic acid from CO₂ hydrogenation. Here, a highly efficient catalytic system that transforms CO₂ to formic acid without the need of strong bases is demonstrated, avoiding the formation of formate salts.

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.

Hydrogenation reactions catalyzed by HN(CH2CH2PR2)2-ligated copper complexes

Hydrogenation of aldehydes and ketones can be catalyzed by a PNP-ligated copper hydride that is accessible from the copper borohydride or bromide complex or the copper hydride cluster.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Chiral-at-Metal: Iridium(III) Tetrazole Complexes With Proton-Responsive P-OH Groups for CO2 Hydrogenation

A rise in atmospheric CO2 levels, following years of burning fossil fuels, has brought about increase in global temperatures and climate change due to the greenhouse effect. As such, recent efforts in addressing this problem have been directed to the use of CO2 as a non-expensive and non-toxic single carbon, C1, source for making chemical products. Herein, we report on the use of tetrazolyl complexes as catalyst precursors for hydrogenation of CO2. Specifically, tetrazolyl compounds bearing P-S bonds have been synthesized with the view of using these as P∧N bidentate tetrazolyl ligands (1-3) that can coordinate to iridium(III), thereby forming heteroatomic five-member complexes. Interestingly, reacting the P,N'-bidentate tetrazolyl ligands with [Ir(C5 Me 5)Cl 2]2 led to serendipitous isolation of chiral-at-metal iridium(III) half-sandwich complexes (7-9) instead. Complexes 7-9 were obtained via prior formation of non-chiral iridium(III) half-sandwich complexes (4-6). The complexes undergo prior P-S bond heterolysis of the precursor ligands, which then ultimately results in new half-sandwich iridium(III) complexes featuring monodentate phosphine co-ligands with proton-responsive P-OH groups. Conditions necessary to significantly affect the rate of P-S bond heterolysis in the precursor ligand and the subsequent coordination to iridium have been reported. The complexes served as catalyst precursors and exhibited activity in CO2 and bicarbonate hydrogenation in excellent catalytic activity, at low catalyst loadings (1 μmol or 0.07 mol% with respect to base), producing concentrated formate solutions (ca 180 mM) exclusively. Catalyst precursors with proton-responsive P-OH groups were found to influence catalytic activity when present as racemates, while ease of dissociation of the ligand from the iridium center was observed to influence activity in spite of the presence of electron-donating ligands. A test for homogeneity indicated that hydrogenation of CO2 proceeded by homogeneous means. Subsequently, the mechanism of the reaction by the iridium(III) catalyst precursors was studied using 1H NMR techniques. This revealed that a chiral-at-metal iridium hydride species generated in situ served as the active catalyst.

Formylation of Amines by CO2 Hydrogenation Using Preformed Co(II)/Nickel(II) Complexes

Preparation of formamides by CO₂ hydrogenation requires an efficient catalyst and temperatures around 100 °C or higher, but most catalysts reported so far incorporate rare and toxic precious metals. Five cobalt(II) or nickel(II) complexes with dmpe or PNN (dmpe = 1,2-bis(dimethylphosphino)ethane; PNN = [(2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine) have been evaluated as precatalysts for the hydrogenation of CO₂ to prepare formamides from the corresponding secondary amines. The most active catalyst for these reactions was found to be [NiCl₂(dmpe)] in DMSO, producing dimethylformamide (DMF) from CO₂, H₂, and dimethylamine in up to 6300 TON, the highest activity reported for this reaction with an abundant metal-phosphine complex.

Controlling the Product Platform of Carbon Dioxide Reduction: Adaptive Catalytic Hydrosilylation of CO2 Using a Molecular Cobalt(II) Triazine Complex

The catalytic reduction of carbon dioxide (CO2 ) is considered a major pillar of future sustainable energy systems and chemical industries based on renewable energy and raw materials. Typically, catalysts and catalytic systems are transforming CO2 preferentially or even exclusively to one of the possible reduction levels and are then optimized for this specific product. Here, we report a cobalt-based catalytic system that enables the adaptive and highly selective transformation of carbon dioxide individually to either the formic acid, the formaldehyde, or the methanol level, demonstrating the possibility of molecular control over the desired product platform.

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH₃ molecules undergo MLC binding with a triaryl noninnocent N₂S₂ ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N₂S₂ ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Cyclometalated Ruthenium Pincer Complexes as Catalysts for the α-Alkylation of Ketones with Alcohols

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H⁻ as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

A highly active copper catalyst for the hydrogenation of carbon dioxide to formate under ambient conditions

Carbon dioxide (CO2) is an important reactant and can be used for the syntheses of various types of industrially important chemicals. Hence, investigation concerning the conversion of CO2 into valuable energy-rich chemicals is an important and current topic in molecular catalysis. Recent research on molecular catalysts has led to improved rates for conversion of CO2 to energy-rich products such as formate, but the catalysts based on first-row transition metals are underdeveloped. Copper(i) complexes containing the 1,1'-bis(di-tert-butylphosphino) ferrocene ligand were found to promote the catalytic hydrogenation of CO2 to formate in the presence of DBU as the base, where the catalytic conversion of CO2via hydrogenation is achieved using in situ gaseous H2 (granulated tin metal and concentrated HCl) to produce valuable energy-rich chemicals, and therefore it is a promising, safe and simple strategy to conduct reactions under ambient pressure at room temperature. Towards this goal, we report an efficient copper(i) complex based catalyst [CuI(dtbpf)] to achieve ambient-pressure CO2 hydrogenation catalysis for generating the formate salt (HCO2-) with turnover number (TON) values of 326 to 1.065 × 105 in 12 to 48 h of reaction at 25 °C to 80 °C. The outstanding catalytic performance of [CuI(dtbpf)] makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.

The potential of d6 non-noble metal NHC catalysts for carbon dioxide hydrogenation: group and row effects

Group VIB NHC-complexes as promising non-noble catalysts for CO₂ hydrogenation benefiting by the weak electronegativity and low oxidation state.

Catalytic hydrogenation of CO2 at a structurally rigidified cobalt center

Catalytic hydrogenation of CO₂ occurs at a cobalt center supported by a rigidified PNP ligand revealing higher catalytic performance.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation

The reversible activation of H₂ via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H₂ via an MLC pathway to the widely used iron catalyst [(^iPrPNP)FeH(CO)] (1) (^iPrPNP = N(CH₂CH₂PⁱPr₂)₂^-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H₂ using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H₂ activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H₂ activation. The results of this study provide guidance about the design of catalysts and additives to promote H₂ activation via an MLC pathway.