Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide

Highly selective hydrogenation of amides catalysed by a molybdenum pincer complex: scope and mechanism

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides.

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides. Among the tested catalysts, Mo-1a proved to be particularly well suited for the selective C–N hydrogenolysis of N-methylated formanilides. Notably, high chemoselectivity was observed in the presence of certain reducible groups including even other amides. The general catalytic performance as well as selectivity issues could be rationalized taking an anionic Mo(0) as the active species. The interplay between the amide C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O reduction and the catalyst poisoning by primary amides accounts for the selective hydrogenation of N-methylated formanilides. The catalyst resting state was found to be a Mo–alkoxo complex formed by reaction with the alcohol product. This species plays two opposed roles – it facilitates the protolytic cleavage of the C–N bond but it encumbers the activation of hydrogen.

Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol

Methanol synthesis from syngas (CO/H₂ mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200-300 °C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temperature/low-pressure (120-150 °C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mechanical simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover numbers (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.

Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads

The capture of CO₂ from concentrated emission sources as well as from air represents a process of paramount importance in view of the increasing CO₂ concentration in the atmosphere and its associated negative consequences on the biosphere. Once captured using various technologies, CO₂ is desorbed and compressed for either storage (carbon capture and storage (CCS)) or production of value-added products (carbon capture and utilization (CCU)). Among various products that can be synthesized from CO₂, methanol and formic acid are of high interest because they can be used directly as fuels or to generate H₂ on demand at low temperatures (<100 °C), making them attractive hydrogen carriers (12.6 and 4.4 wt % H₂ in methanol and formic acid, respectively). Methanol is already produced in huge quantities worldwide (100 billion liters annually) and is also a raw material for many chemicals and products, including formaldehyde, dimethyl ether, light olefins, and gasoline. The production of methanol through chemical recycling of captured CO₂ is at the heart of the so-called "methanol economy" that we have proposed with the late Prof. George Olah at our Institute. Recently, there has been significant progress in the low-temperature synthesis of formic acid (or formate salts) and methanol from CO₂ and H₂ using homogeneous catalysts. Importantly, several studies have combined CO₂ capture and hydrogenation, where captured CO₂ (including from air) was directly utilized to produce formate and CH₃OH without requiring energy intensive desorption and compression steps. This Account centers on that topic. A key feature in the combined CO₂ capture and conversion studies reported to date for the synthesis of formic acid and methanol is the use of an amine or alkali-metal hydroxide base for capturing CO₂, which can assist the homogeneous catalysts in the hydrogenation step. We start this Account by examining the combined processes where CO₂ is captured in amine solutions and converted to alkylammonium formate salts. The effect of amine basicity on the reaction rate is discussed along with catalyst recycling schemes. Next, methanol synthesis by this combined process, with amines as capturing agents, is explored. We also examine the system developments for effective catalyst and amine recycling in this process. We next go through the effect of catalyst molecular structure on methanol production while elucidating the main deactivating pathway involving carbonylation of the metal center. The recent advances in first-row transition metal catalysts for this process are also mentioned. Subsequently, we discuss the capture of CO₂ using hydroxide bases and conversion to formate salts. The regeneration of the hydroxide base (NaOH or KOH) at low temperatures (80 °C) in cation-conducting direct formate fuel cells is presented. Finally, we review the challenges in the yet unreported integrated CO₂ capture by hydroxide bases and conversion to methanol process.

Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements

Amines were immobilized onto solid supports and employed for tandem CO₂ capture and conversion to CH₃ OH using homogeneous hydrogenation catalysts. The hydrogenation proceeded through the formation of formamide intermediates. After hydrogenation, the immobilized amines were easily filtered and collected to be reused. The catalyst and methanol were recovered from the filtrate. Covalently-attached (to polymer support or silica) amine functionalities displayed the highest recycling potential with almost no leaching under the hydrogenation reaction conditions. Using polyethylenimine grafted onto a solid-silica support, the catalyst and amine were successfully recycled, and CO₂ (either pure or from the air) was efficiently captured and converted to CH₃ OH over multiple cycles.

Selective Hydroboration of Carboxylic Acids with a Homogeneous Manganese Catalyst

Catalytic reduction of carboxylic acid to the corresponding alcohol is a challenging task of great importance for the production of a variety of value-added chemicals. Herein, a manganese-catalyzed chemoselective hydroboration of carboxylic acids has been developed with a high turnover number (>99 000) and turnover frequency (>2000 h^-1) at 25 °C. This method displayed tolerance of electronically and sterically differentiated substrates with high chemoselectivity. Importantly, aliphatic long-chain fatty acids, including biomass-derived compounds, can efficiently be reduced. Mechanistic studies revealed that the reaction occurs through the formation of active manganese-hydride species via an insertion and bond metathesis type mechanism.

Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(i) complexes bearing triazole ligands

Manganese(i) complexes bearing readily accessible triazole ligands are effective catalysts for the hydrosilylation of carbonyl and carboxyl compounds.

Additive-free cobalt-catalysed hydrogenation of carbonates to methanol and alcohols

Homogeneously cobalt-catalyzed hydrogenation of cyclic and acyclic carbonates: beneficial effects of 2,2,2-trifluoroethanol and triphos-derivatives.

CO2 activation by manganese pincer complexes through different modes of metal–ligand cooperation

Activation of CO₂ by manganese pincer complexes using two different modes of metal–ligand cooperativity (amido/amino mode and dearomatization/aromatization mode) is reported.

Theoretical insights into CO2 hydrogenation to methanol by a Mn–PNP complex

Unravelling the role of an amide intermediate in co-catalyst-based sequential CO₂ hydrogenation reaction to methanol.

Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation

Rational design using kinetic studies has led to a 3-fold-increase in the reaction-rates compared to an already-promising lead catalyst for the reduction of ketones and esters.

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.

Hydrogenation of Nitriles and Ketones Catalyzed by an Air-Stable Bisphosphine Mn(I) Complex

Efficient hydrogenations of nitriles and ketones with molecular hydrogen catalyzed by a well-defined bench-stable bisphosphine Mn(I) complex are described. These reactions are environmentally benign and atomically economic, implementing an inexpensive, earth-abundant nonprecious metal catalyst. A range of aromatic and aliphatic nitriles and ketones were efficiently converted into primary amines and alcohols, respectively, in good to excellent yields. The hydrogenation of nitriles proceeds at 100 °C with catalyst loading of 2 mol % and 20 mol % base ( t-BuOK), while the hydrogenation of ketones takes place already at 50 °C, with a catalyst loading of 1 mol % and 5 mol % of base. In both cases, a hydrogen pressure of 50 bar was applied.

Manganese-Catalyzed α-Alkylation of Ketones, Esters, and Amides Using Alcohols

Herein we report the manganese-catalyzed C–C bond-forming reactions via α-alkylation of ketones, amides, and esters, using primary alcohols. β-Alkylation of secondary alcohols by primary alcohols to obtain α-alkylated ketones is also reported. The reactions are catalyzed by a (ⁱPr-PNP)Mn(H)(CO)₂ pincer complex under mild conditions in the presence of (catalytic) base liberating water (and H₂ in the case of secondary alcohol alkylation) as the sole byproduct.

Aminotriazole Mn(I) Complexes as Effective Catalysts for Transfer Hydrogenation of Ketones

A catalytic system based on complexes comprising abundant and cheap manganese together with readily available aminotriazole ligands is reported. The new Mn(I) complexes are catalytically competent in transfer hydrogenation of ketones with 2-propanol as hydrogen source. The reaction proceeds under mild conditions at 80 °C for 20 h with 3 % of catalyst loading using either KO t Bu or NaOH as base. Good to excellent yields were obtained for a wide substrate scope with broad functional group tolerance. The obtained results by varying the substitution pattern of the ligand are consistent with an out-sphere mechanism for the H-transfer.

A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide

A highly efficient recyclable system for capture and subsequent conversion of CO₂ to formate salts is reported that utilizes aqueous inorganic hydroxide solutions for CO₂ capture along with homogeneous pincer catalysts for hydrogenation. The produced aqueous solutions of formate salts are directly utilized, without any purification, in a direct formate fuel cell to produce electricity and regenerate the hydroxide base, achieving an overall carbon-neutral cycle. The catalysts and organic solvent are recycled by employing a biphasic solvent system (2-MTHF/H₂O) with no significant decrease in turnover frequency (TOF) over five cycles. Among different hydroxides, NaOH and KOH performed best in tandem CO₂ capture and conversion due to their rapid rate of capture, high formate conversion yield, and high catalytic TOF to their corresponding formate salts. Among various catalysts, Ru- and Fe-based PNP complexes were the most active for hydrogenation. The extremely low vapor pressure, nontoxic nature, easy regenerability, and high reactivity of NaOH/KOH toward CO₂ make them ideal for scrubbing CO₂ even from low-concentration sources-such as ambient air-and converting it to value-added products.

Catalytic Hydrogenation of Cyclic Carbonates using Manganese Complexes

Catalytic hydrogenation of cyclic carbonates to diols and methanol was achieved using a molecular catalyst based on earth-abundant manganese. The complex [Mn(CO)2 (Br)[HN(C2 H4 Pi Pr2 )2 ] 1 comprising commercially available MACHO ligand is an effective pre-catalyst operating under relatively mild conditions (T=120 °C, p(H2 )=30-60 bar). Upon activation with NaOt Bu, the formation of coordinatively unsaturated complex [Mn(CO)2 [N(C2 H4 Pi Pr2 )2 )] 5 was spectroscopically verified, which confirmed a kinetically competent intermediate. With the pre-activated complex, turnover numbers up to 620 and 400 were achieved for the formation of the diol and methanol, respectively. Stoichiometric reactions under catalytically relevant conditions provide insight into the stepwise reduction form the CO2 level in carbonates to methanol as final product.

Hydrogenation of CO2 -Derived Carbonates and Polycarbonates to Methanol and Diols by Metal-Ligand Cooperative Manganese Catalysis

The first base-metal-catalysed hydrogenation of CO₂ -derived carbonates to alcohols is presented. The reaction proceeds under mild conditions in the presence of a well-defined manganese complex with a loading as low as 0.25 mol %. The non-precious-metal homogenous catalytic system provides an indirect route for the conversion of CO₂ into methanol with the co-production of value-added (vicinal) diols in yields of up to 99 %. Experimental and computational studies indicate a metal-ligand cooperative catalysis mechanism.

Coupling Glucose Dehydrogenation with CO2 Hydrogenation by Hydrogen Transfer in Aqueous Media at Room Temperature

Conversion of CO₂ into value-added chemicals and fuels provides a direct solution to reduce excessive CO₂ in the atmosphere. Herein, a novel catalytic reaction system is presented by coupling the dehydrogenation of glucose with the hydrogenation of a CO₂ -derived salt, ammonium carbonate, in an ethanol-water mixture. For the first time, the hydrogenation of CO₂ to formate by glucose has been achieved under ambient conditions. Under the optimal reaction conditions, the highest yield of formate reached approximately 46 %. We find that the apparent pH value in the ethanol-water mixture plays a central role in determining the performance of the hydrogen-transfer reaction. Based on the ¹³ C NMR and ESI-MS results, a possible pathway of the coupled glucose dehydrogenation and CO₂ hydrogenation reactions was proposed.