Oxidation of Organic Compounds Using Water as the Oxidant with H2 Liberation Catalyzed by Molecular Metal Complexes.

Oxidation reactions of organic compounds play a central role in both industrial chemical and material synthesis as well as in fine chemical and pharmaceutical synthesis. While traditional laboratory-scale oxidative syntheses have relied on the use of strong oxidizers, modern large-scale oxidation processes preferentially utilize air or pure O2 as an oxidant, with other oxidants such as hydrogen peroxide, nitric acid, and aqueous chlorine solution also being used in some processes. The use of molecular oxygen or air as an oxidant has been very attractive in recent decades because of the abundance of air and the lack of wasteful byproduct generation. Nevertheless, the use of high-pressure air or, in particular, pure oxygen can lead to serious safety concerns with improper handling and also necessitates the use of sophisticated high-pressure reactors for the processes.Several research groups, including ours, have investigated in recent times the possibility of carrying out catalytic oxidation reactions using water as the formal oxidant, with no added conventional oxidants. Along with the abundant availability of water, these processes also generate dihydrogen gas as the reaction coproduct, which is a highly valuable fuel. Several well-defined molecular metal complexes have been reported in recent years to catalyze these unusual oxidative reactions with water. A ruthenium bipyridine-based PNN pincer complex was reported by us to catalyze the oxidation of primary alcohols to carboxylate salts with alkaline water along with H2 liberation, followed by reports by other groups using other complexes as catalysts. At the same time, ruthenium-, iridium-, and rhodium-based complexes have been reported to catalyze aldehyde oxidation to carboxylic acids using water. Our group has combined the catalytic aqueous alcohol and aldehyde oxidation activity of a ruthenium complex to achieve the oxidation of biomass-derived renewable aldehydes such as furfural and 5-hydroxymethylfurfural (HMF) to furoic acid and furandicarboxylic acid (FDCA), respectively, using alkaline water as the oxidant, liberating H2. Ruthenium complexes with an acridine-based PNP ligand have also been employed by our group for the catalytic oxidation of amines to the corresponding lactams, or to carboxylic acids via a deaminative route, using water. Similarly, we also reported molecular complexes for the catalytic Markovnikov oxidation of alkenes to ketones using water, similar to Wacker-type oxidation, which, however, does not require any terminal oxidant and produces H2 as the coproduct. At the same time, the oxidation of enol ethers to the corresponding esters with water has also been reported. This account will highlight these recent advances where water was used as an oxidant to carry out selective oxidation reactions of organic compounds, catalyzed by well-defined molecular complexes, with H2 liberation. The oxidation of alcohols, aldehydes, amines, alkenes, and enol ethers will be discussed to provide an outlook toward other functional groups' oxidation. We hope that this will aid researchers in devising other oxidative dehydrogenative catalytic systems using water, complementing traditional oxidative processes involving strong oxidants and molecular oxygen.

Catalytic Furfural/5-Hydroxymethyl Furfural Oxidation to Furoic Acid/Furan-2,5-dicarboxylic Acid with H2 Production Using Alkaline Water as the Formal Oxidant.

Furfural and 5-hydroxymethyl furfural (HMF) are abundantly available biomass-derived renewable chemical feedstocks, and their oxidation to furoic acid and furan-2,5-dicarboxylic acid (FDCA), respectively, is a research area with huge prospective applications in food, cosmetics, optics, and renewable polymer industries. Water-based oxidation of furfural/HMF is a lucrative approach for simultaneous generation of H2 and furoic acid/FDCA. However, this process is currently limited to (photo)electrochemical methods that can be challenging to control, improve, and scale up. Herein, we report well-defined ruthenium pincer catalysts for direct homogeneous oxidation of furfural/HMF to furoic acid/FDCA, using alkaline water as the formal oxidant while producing pure H2 as the reaction byproduct. Mechanistic studies indicate that the ruthenium complex not only catalyzes the aqueous oxidation but also actively suppresses background decomposition by facilitating initial Tishchenko coupling of substrates, which is crucial for reaction selectivity. With further improvement, this process can be used in scaled-up facilities for a simultaneous renewable building block and fuel production.

Efficient Base-Free Aqueous Reforming of Methanol Homogeneously Catalyzed by Ruthenium Exhibiting a Remarkable Acceleration by Added Catalytic Thiol.

Production of H2 by methanol reforming is of particular interest due the low cost, ready availability, and high hydrogen content of methanol. However, most current methods either require very high temperatures and pressures or strongly rely on the utilization of large amounts of base. Here we report an efficient, base-free aqueous-phase reforming of methanol homogeneously catalyzed by an acridine-based ruthenium pincer complex, the activity of which was unexpectedly improved by a catalytic amount of a thiol additive. The reactivity of this system is enhanced by nearly 2 orders of magnitude upon addition of the thiol, and it can maintain activity for over 3 weeks, achieving a total H2 turnover number of over 130 000. On the basis of both experimental and computational studies, a mechanism is proposed which involves outer-sphere dehydrogenations promoted by a unique ruthenium complex with thiolate as an assisting ligand. The current system overcomes the need for added base in homogeneous methanol reforming and also highlights the unprecedented acceleration of catalytic activity of metal complexes achieved by the addition of a catalytic amount of thiol.

Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol.

The first example of an alkali hydroxide-based system for CO2 capture and conversion to methanol has been established. Bicarbonate and formate salts were hydrogenated to methanol with high yields in a solution of ethylene glycol. In an integrated one-pot system, CO2 was efficiently captured by an ethylene glycol solution of the base and subsequently hydrogenated to CH3OH at relatively mild temperatures (100-140 °C) using Ru-PNP catalysts. The produced methanol can be easily separated by distillation. Hydroxide base regeneration at low temperatures was observed for the first time. Finally, CO2 capture from ambient air and hydrogenation to CH3OH was demonstrated. We postulate that the high capture efficiency and stability of hydroxide bases make them superior to existing amine-based routes for direct air capture and conversion to methanol in a scalable process.

Metal-Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes.

A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal-ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N-H and H-H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facilitating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.

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.

Catalytic Homogeneous Hydrogenation of CO to Methanol via Formamide.

A novel amine-assisted route for low temperature homogeneous hydrogenation of CO to methanol is described. The reaction proceeds through the formation of formamide intermediates. The first amine carbonylation part is catalyzed by K3PO4. Subsequently, the formamides are hydrogenated in situ to methanol in the presence of a commercially available ruthenium pincer complex as a catalyst. Under optimized reaction conditions, CO (up to 10 bar) was directly converted to methanol in high yield and selectivity in the presence of H2 (70 bar) and diethylenetriamine. A maximum TON of 539 was achieved using the catalyst Ru-Macho-BH. The high yield, selectivity, and TONs obtained for methanol production at low reaction temperature (145 °C) could make this process an attractive alternative over the traditional high temperature heterogeneous catalysis.

Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol.

Amine-assisted homogeneous hydrogenation of CO2 to methanol is one of the most effective approaches to integrate CO2 capture with its subsequent conversion to CH3OH. The hydrogenation typically proceeds in two steps. In the first step the amine is formylated via an in situ formed alkylammonium formate salt (with consumption of 1 equiv of H2). In the second step the generated formamide is further hydrogenated with 2 more equiv of H2 to CH3OH while regenerating the amine. In the present study, we investigated the effect of molecular structure of the ruthenium pincer catalysts and the amines that are critical for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNP R(CO)] (R = Ph/ i-Pr/Cy/ t-Bu) for both amine formylation and formamide hydrogenation, only catalyst Ru-Macho (R = Ph) provided a high methanol yield after both steps were performed simultaneously in one pot. Among various amines, only (di/poly)amines were effective in assisting Ru-Macho for methanol formation. A catalyst deactivation pathway was identified, involving the formation of ruthenium biscarbonyl monohydride cationic complexes [RuHPNP R(CO)2]+, whose structures were unambiguously characterized and whose reactivities were studied. These reactivities were found to be ligand-dependent, and a trend could be established. With Ru-Macho, the biscarbonyl species could be converted back to the active species through CO dissociation under the reaction conditions. The Ru-Macho biscarbonyl complex was therefore able to catalyze the hydrogenation of in situ formed formamides to methanol. Complex Ru-Macho-BH was also highly effective for this conversion and remained active even after 10 days of continuous reaction, achieving a maximum turnover number (TON) of 9900.

Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy.

Herein we report an efficient and recyclable system for tandem CO2 capture and hydrogenation to methanol. After capture in an aqueous amine solution, CO2 is hydrogenated in high yield to CH3OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy separation and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol producibility of the first cycle retained, along with 95% of catalyst activity after four cycles. CO2 from dilute sources such as air can also be converted to CH3OH using this route. We postulate that the CO2 capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.

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.

Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol.

A novel hydrogen storage system based on the hydrogen release from catalytic dehydrogenative coupling of methanol and 1,2-diamine is demonstrated. The products of this reaction, N-formamide and N,N'-diformamide, are hydrogenated back to the free amine and methanol by a simple hydrogen pressure swing. Thus, an efficient one-pot hydrogen carrier system has been developed. The H2 generating step can be termed as "amine reforming of methanol" in analogy to the traditional steam reforming. It acts as a clean source of hydrogen without concurrent production of CO2 (unlike steam reforming) or CO (by complete methanol dehydrogenation). Therefore, a carbon neutral cycle is essentially achieved where no carbon capture is necessary as the carbon is trapped in the form of formamide (or urea in the case of primary amine). In theory, a hydrogen storage capacity as high as 6.6 wt % is achievable. Dehydrogenative coupling and the subsequent amide hydrogenation proceed with good yields (90% and >95% respectively, with methanol and N,N'-dimethylethylenediamine as dehydrogenative coupling partners).

Sustainable catalysis with fluxional acridine-based PNP pincer complexes.

Because of the widespread use of fossil fuels and the resulting global warming, development of sustainable catalytic transformations is now more important than ever to obtain our desired fuels and building materials with the least carbon footprint and waste production. Many sustainable (de)hydrogenation reactions, including CO2 reduction, H2 carrier systems, and others, have been reported using molecular pincer complexes. A specific subset of pincer complexes containing a central acridine donor with flanking CH2PR2 ligands, known as acridine-based PNP pincer complexes, exhibit special reactivities that are not imitable by other PNP pincer complexes such as pyridine-based or (R2PCH2CH2)2NH type ligands. The goal of this article is to highlight the unique reactivities of acridine-based complexes and then investigate how these reactivities allow these complexes to catalyse many sustainable reactions that traditional pincer complexes cannot catalyse. To that end, we will initially go over the synthesis and structural features of acridine complexes, such as the labile coordination of the central N donor and the observed fac-mer fluxionality. Following that, distinct reactivity patterns of acridine-based complexes including their reactivity with acids and water will be discussed. Finally, we will discuss the reaction systems that have been developed with acridine complexes thus far, including the notable selective transformations of primary alcohols to primary amines using ammonia, N-heteroaromatic synthesis from alcohols and ammonia, oxidation reactions with water with H2 liberation, development of H2 carrier systems, and others, and conclude the article with future possible directions. We hope that the systemic study presented here will aid researchers in developing further sustainable reactions based on the unique acridine-based pincer complexes.

Dehydrogenative ester synthesis from enol ethers and water with a ruthenium complex catalyzing two reactions in synergy.

We report the dehydrogenative synthesis of esters from enol ethers using water as the formal oxidant, catalyzed by a newly developed ruthenium acridine-based PNP(Ph)-type complex. Mechanistic experiments and density functional theory (DFT) studies suggest that an inner-sphere stepwise coupled reaction pathway is operational instead of a more intuitive outer-sphere tandem hydration-dehydrogenation pathway.

Highly Efficient Additive-Free Dehydrogenation of Neat Formic Acid.

Formic acid (FA) is a promising hydrogen carrier which can play an instrumental role in the overall implementation of a hydrogen economy. In this regard, it is important to generate H2 gas from neat FA without any solvent/additive, for which existing systems are scarce. Here we report the remarkable catalytic activity of a ruthenium 9H-acridine pincer complex for this process. The catalyst is unusually stable and robust in FA even at high temperatures and can catalyse neat FA dehydrogenation for over a month, with a total turnover number of 1,701,150, while also generating high H2/CO2 gas pressures (tested up to 100 bars). Mechanistic investigations and DFT studies are conducted to fully understand the molecular mechanism to the process. Overall, the high activity, stability, selectivity, simplicity and versatility of the system to generate a CO-free H2/CO2 gas stream and high pressure from neat FA makes it promising for large-scale implementation.

Near-Ambient-Temperature Dehydrogenative Synthesis of the Amide Bond: Mechanistic Insight and Applications.

The current existing methods for the amide bond synthesis via acceptorless dehydrogenative coupling of amines and alcohols all require high reaction temperatures for effective catalysis, typically involving reflux in toluene, limiting their potential practical applications. Herein, we report a system for this reaction that proceeds under mild conditions (reflux in diethyl ether, boiling point 34.6 °C) using ruthenium PNNH complexes. The low-temperature activity stems from the ability of Ru-PNNH complexes to activate alcohol and hemiaminals at near-ambient temperatures through the assistance of the terminal N-H proton. Mechanistic studies reveal the presence of an unexpected aldehyde-bound ruthenium species during the reaction, which is also the catalytic resting state. We further utilize the low-temperature activity to synthesize several simple amide bond-containing commercially available pharmaceutical drugs from the corresponding amines and alcohols via the dehydrogenative coupling method.

Manganese-Pincer-Catalyzed Nitrile Hydration, α-Deuteration, and α-Deuterated Amide Formation via Metal Ligand Cooperation.

A simple and efficient system for the hydration and α-deuteration of nitriles to form amides, α-deuterated nitriles, and α-deuterated amides catalyzed by a single pincer complex of the earth-abundant manganese capable of metal-ligand cooperation is reported. The reaction is selective and tolerates a wide range of functional groups, giving the corresponding amides in moderate to good yields. Changing the solvent from tert-butanol to toluene and using D2O results in formation of α-deuterated nitriles in high selectivity. Moreover, α-deuterated amides can be obtained in one step directly from nitriles and D2O in THF. Preliminary mechanistic studies suggest the transformations contributing toward activation of the nitriles via a metal-ligand cooperative pathway, generating the manganese ketimido and enamido pincer complexes as the key intermediates for further transformations.

Selective Room-Temperature Hydrogenation of Amides to Amines and Alcohols Catalyzed by a Ruthenium Pincer Complex and Mechanistic Insight.

We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols. Compared with most previous reports for this transformation, which use high temperatures (typically, 100-200 °C) and H2 pressures (10-100 bar), this system proceeds under extremely mild conditions (RT, 5-10 bar of H2). The hydrogenation is catalyzed by well-defined ruthenium-PNNH pincer complexes (0.5 mol %) with potential dual modes of metal-ligand cooperation. An unusual Ru-amidate complex was formed and crystallographically characterized. Mechanistic investigations indicate that the room-temperature hydrogenation proceeds predominantly via the Ru-N amido/amine metal-ligand cooperation.

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.

Traversable wormholes with arbitrarily small energy condition violations.

Traversable wormholes necessarily require violations of the averaged null energy condition, this being the definition of "exotic matter." However, the theorems which guarantee the energy condition violation are remarkably silent when it comes to making quantitative statements regarding the "total amount" of energy condition violating matter in the spacetime. We develop a suitable measure for quantifying this notion and demonstrate the existence of spacetime geometries containing traversable wormholes that are supported by arbitrarily small quantities of exotic matter.