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

Metal-Ligand Cooperation with Thiols as Transient Cooperative Ligands: Acceleration and Inhibition Effects in (De)Hydrogenation Reactions

ConspectusOver the past two decades, we have developed a series of pincer-type transition metal complexes capable of activating strong covalent bonds through a mode of reactivity known as metal-ligand cooperation (MLC). In such systems, an incoming substrate molecule simultaneously interacts with both the metal center and ligand backbone, with one part of the molecule reacting at the metal center and another part at the ligand. The majority of these complexes feature pincer ligands with a pyridine core, and undergo MLC through reversible dearomatization/aromatization of this pyridine moiety. This MLC platform has enabled us to perform a variety of catalytic dehydrogenation, hydrogenation, and related reactions, with high efficiency and selectivity under relatively mild conditions.In a typical catalytic complex that operates through MLC, the cooperative ligand remains coordinated to the metal center throughout the entire catalytic process, and this complex is the only catalytic species involved in the reaction. As part of our ongoing efforts to develop new catalytic systems featuring MLC, we have recently introduced the concept of transient cooperative ligand (TCL), i.e., a ligand that is capable of MLC when coordinated to a metal center, but the coordination of which is reversible rather than permanent. We have thus far employed thiol(ate)s as TCLs, in conjunction with an acridanide-based ruthenium(II)-pincer catalyst, and this has resulted in remarkable acceleration and inhibition effects in various hydrogenation and dehydrogenation reactions. A cooperative thiol(ate) ligand can be installed in situ by the simple addition of an appropriate thiol in an amount equivalent to the catalyst, and this has been repeatedly shown to enable efficient bond activation by MLC without the need for other additives, such as base. The use of an ancillary thiol ligand that is not fixed to the pincer backbone allows the catalytic system to benefit from a high degree of tunability, easily implemented by varying the added thiol. Importantly, thiols are coordinatively labile enough under typical catalytic conditions to leave a meaningful portion of the catalyst in its original unsaturated form, thereby allowing it to carry out its own characteristic catalytic activity. This generates two coexisting catalyst populations─one that contains a thiol(ate) ligand and another that does not─and this may lead to different catalytic outcomes, namely, enhancement of the original catalytic activity, inhibition of this activity, or the occurrence of diverging reactivities within the same catalytic reaction mixture. These thiol effects have enabled us to achieve a series of unique transformations, such as thiol-accelerated base-free aqueous methanol reforming, controlled stereodivergent semihydrogenation of alkynes using thiol as a reversible catalyst inhibitor, and hydrogenative perdeuteration of C═C bonds without using D2, enabled by a combination of thiol-induced acceleration and inhibition. We have also successfully realized the unprecedented formation of thioesters through dehydrogenative coupling of alcohols and thiols, as well as the hydrogenation of organosulfur compounds, wherein the cooperative thiol serves as a reactant or product. In this Account, we present an overview of the TCL concept and its various applications using thiols.

Production of hydrogen from alcohols via homogeneous catalytic transformations mediated by molecular transition-metal complexes

Hydrogen obtained from renewable sources such as water and alcohols is regarded as an efficient clean-burning alternative to non-renewable fuels. The use of the so-called bio-H2 regardless of its colour will be a significant step towards achieving global net-zero carbon goals. Challenges still persist however with conventional H2 storage, which include low-storage density and high cost of transportation apart from safety concerns. Global efforts have thus focussed on liquid organic hydrogen carriers (LOHCs), which have shown excellent potential for H2 storage while allowing safer large-scale transformation and easy on-site H2 generation. While water could be considered as the most convenient liquid inorganic hydrogen carrier (LIHC) on a long-term basis, the utilization of alcohols as LOHCs to generate on-demand H2 has tasted instant success. This has helped to draw a road-map of futuristic H2 storage and transportation. The current review brings to the fore the state-of-the-art developments in hydrogen generation from readily available, feed-agnostic bio-alcohols as LOHCs using molecular transition-metal catalysts.

Recent progress in molecular transition metal catalysts for hydrogen production from methanol and formaldehyde

Hydrogen is considered as a potential alternative and sustainable energy carrier, but its safe storage and transportation are still challenging due to its low volumetric energy density. Notably, C1-based substrates, methanol and formaldehyde, containing high hydrogen contents of 12.5 wt% and 6.7 wt%, respectively, can release hydrogen on demand in the presence of a suitable catalyst. Advantageously, both methanol and aqueous formaldehyde are liquid at room temperature, and hence can be stored and transported considerably more safely than hydrogen gas. Moreover, these C1-based substrates can be produced from biomass waste and can also be regenerated from CO2, a greenhouse gas. In this review, the recent progress in hydrogen production from methanol and formaldehyde over noble to non-noble metal complex-based molecular transition metal catalysts is extensively reviewed. This review also focuses on the critical role of the structure-activity relationship of the catalyst in the dehydrogenation pathway.

Hydrogenative alkene perdeuteration aided by a transient cooperative ligand

Deuterogenation of unsaturated organic compounds is an attractive route for installing C(sp3)-D bonds, but the existing methods typically use expensive D2 and introduce only two deuterium atoms per unsaturation. Herein we report the hydrogenative perdeuteration of alkenes using readily available H2 and D2O instead of D2, catalysed by an acridanide-based ruthenium pincer complex and resulting in the incorporation of up to 4.9 D atoms per C=C double bond in a single synthetic step. Importantly, adding a catalytic amount of thiol, which serves as a transient cooperative ligand, ensures the incorporation of deuterium rather than protium by balancing the rates of two sequential deuteration processes. The current method opens an avenue for installing perdeuteroalkyl groups at specific sites from widely available alkenes under mild conditions.

Methanol as a Potential Hydrogen Source for Reduction Reactions Enabled by a Commercial Pt/C Catalyst

Catalytic reduction reactions using methanol as a transfer hydrogenating agent is gaining significant attention because this simple alcohol is inexpensive and produced on a bulk scale. Herein, we report the catalytic utilization of methanol as a hydrogen source for the reduction of different functional organic compounds such as nitroarenes, olefins, and carbonyl compounds. The key to the success of this transformation is the use of a commercially available Pt/C catalyst, which enabled the transfer hydrogenation of a series of simple and functionalized nitroarenes-to-anilines, alkenes-to-alkanes, and aldehydes-to-alcohols using methanol as both the solvent and hydrogen donor. The practicability of this Pt-based protocol is showcased by demonstrating catalyst recycling and reusability as well as reaction upscaling. In addition, the Pt/C catalytic system was also adaptable for the N-methylation and N-alkylation of anilines via the borrowing hydrogen process. This work provides a simple and flexible approach to prepare a variety of value-added products from readily available methanol, Pt/C, and other starting materials.

Sustainable amidation through acceptorless dehydrogenative coupling by pincer-type catalysts: recent advances

The amide functional group is ubiquitous in living organisms, and is of particular importance in bioactive compounds and pharmaceuticals. Because of the prevalence and significance of the amide bond, considerable efforts have been invested throughout the years in developing new synthetic methodologies for its formation. Nevertheless, amide synthesis still largely relies on variants of the traditional condensation of carboxylic acids and amines, mediated by stoichiometric coupling reagents. This poses a sustainability challenge, since such reactions suffer from unfavorable atom and step economies, involve harmful chemicals and produce chemical waste. Hence, establishing sustainable approaches to amide synthesis is of great importance. Over the last two decades, we have developed homogeneous catalytic reactions for sustainable synthetic transformations, primarily based on transition metal complexes of pincer ligands. A considerable portion of these efforts has been devoted to acceptorless dehydrogenative coupling, including that of alcohols and amines through ruthenium-catalyzed reactions. These latter processes generate amides without resorting to coupling reagents and typically produce no waste, with their only byproduct being H2 gas, which is itself a valuable resource. In the present review, we chronicle our progress in this area of research since 2014. This includes the use of water and ammonia as amidation reagents, expanding the scope of amidation substrates and target amides, achieving milder reaction conditions, development of amidation-based liquid organic hydrogen carrier systems, and introduction of manganese-based catalysts.

Selective oxidation of silanes into silanols with water using [MnBr(CO)5] as a precatalyst

The development of earth-abundant catalysts for the selective conversion of silanes to silanols with water as an oxidant generating valuable hydrogen as the only by-product continues to be a challenge. Here, we demonstrate that [MnBr(CO)₅] is a highly active precatalyst for this reaction, operating under neutral conditions and avoiding the undesired formation of siloxanes. As a result, a broad substrate scope, including primary and secondary silanes, could be converted to the desired products. The turnover performances of the catalyst were also examined, yielding a maximum TOF of 4088 h^-1. New light was shed on the debated mechanism of the interaction between [MnBr(CO)₅] and Si-H bonds based on the reaction kinetics (including KIEs of PhMe₂SiD and D₂O) and spectroscopic techniques (FT-IR, GC-TCD, ¹H-, ²⁹Si-, and ¹³C-NMR). The initial activation of [MnBr(CO)₅] was found to result from the formation of a manganese(i) hydride species and R₃SiBr, and the experimental data are most consistent with a catalytic cycle comprising a cationic tricarbonyl Mn(i) unit as the active framework.

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.

Water as a solvent: transition metal catalyzed dehydrogenation of alcohols going green

The long-established practice of using organic solvents in synthetic chemistry is currently becoming a major focus of environmental alarms as many of the chemical wastes are generated in the form of organic solvents. Recently, various alternative solvents have been recognized by the scientific community, including water, ionic liquids, supercritical fluids, glycerol, polyethylene glycol, etc. Among these alternatives, water is unquestionably an ideal solvent as it is abundant, cheap, non-toxic, and non-flammable. In the last few decades, a breakthrough has been achieved in the field of transition metal-catalyzed dehydrogenation of alcohols and the related chemistry for the sustainable synthesis of a wide range of valuable compounds. Although a large number of reports with new potential are published every year following this alcohol dehydrogenation strategy, the utilization of water as a solvent in alcohol dehydrogenation and related coupling reactions is yet to be highlighted properly. This review summarizes the advances in metal-catalyzed dehydrogenative functionalization of alcohols using water as a solvent.

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Catalytic semihydrogenation of internal alkynes using H₂ is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H₂-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

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.

Experimental Assessment of Perhydro-Dibenzyltoluene Dehydrogenation Reaction Kinetics in a Continuous Flow System for Stable Hydrogen Supply

The development of alternate clean energy resources is among the most pressing issues in the energy sector in order to preserve the global natural environment. One of the ideal candidates is the utilization of hydrogen as a primary fuel in lieu of fossil fuels. It can be safely stored in liquid organic hydrogen carrier (LOHC) materials and recovered on demand. A uniform supply of hydrogen is essential for power production systems for their smooth operation. This study was conducted to determine the operating conditions of the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT) to ensure that hydrogen supply in a continuous flow reactor remains stable over a wide range of temperatures. The hydrogen flow rate from the dehydrogenation reaction was measured and correlated with the degree of dehydrogenation (DoD) evaluated from the refractive index of reactant liquid samples at various temperatures, WHSV and the initial reactant concentrations. Moreover, a kinetic model is presented holding validity up to a WHSV of 67 h⁻¹. The results acquired present a range for an order of reaction from 2.3 to 2.4 with the required activation energy of 171 kJ/mol.