Homogeneous Reforming of Aqueous Ethylene Glycol to Glycolic Acid and Pure Hydrogen Catalyzed by Pincer-Ruthenium Complexes Capable of Metal-Ligand Cooperation

Half-Sandwich Iridium Complexes: A Recyclable and Stable Catalyst for Dehydrogenation of Alcohols to Carboxylic Acids

A series of acylhydrazone-based N,N-chelate half-sandwich iridium complexes have been synthesized through a facile route in good yields. The dehydrogenation of a series of aromatic and aliphatic primary alcohols to corresponding carboxylic acids has been accomplished catalyzed by the prepared air stable iridium complexes under mild reaction conditions. Carboxylic acids were obtained in high yields under open flask condition with broad substrates and good tolerance to sensitive functional groups. Such a half-sandwich iridium catalyst system exhibited high catalytic activity and stability, and a high TOF of 316.7 h-1 could be achieved with a catalyst loading as low as 0.05 mol %. Furthermore, the sustainable catalyst could be reused at least five times without obviously losing its activity, highlighting its potential application in industry. Molecular structure of iridium complex 1 was confirmed by single-crystal X-ray analysis.

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.

%VBur index and steric maps: from predictive catalysis to machine learning

Steric indices are parameters used in chemistry to describe the spatial arrangement of atoms or groups of atoms in molecules. They are important in determining the reactivity, stability, and physical properties of chemical compounds. One commonly used steric index is the steric hindrance, which refers to the obstruction or hindrance of movement in a molecule caused by bulky substituents or functional groups. Steric hindrance can affect the reactivity of a molecule by altering the accessibility of its reactive sites and influencing the geometry of its transition states. Notably, the Tolman cone angle and %VBur are prominent among these indices. Actually, steric effects can also be described using the concept of steric bulk, which refers to the space occupied by a molecule or functional group. Steric bulk can affect the solubility, melting point, boiling point, and viscosity of a substance. Even though electronic indices are more widely used, they have certain drawbacks that might shift preferences towards others. They present a higher computational cost, and often, the weight of electronics in correlation with chemical properties, e.g. binding energies, falls short in comparison to %VBur. However, it is worth noting that this may be because the steric index inherently captures part of the electronic content. Overall, steric indices play an important role in understanding the behaviour of chemical compounds and can be used to predict their reactivity, stability, and physical properties. Predictive chemistry is an approach to chemical research that uses computational methods to anticipate the properties and behaviour of these compounds and reactions, facilitating the design of new compounds and reactivities. Within this domain, predictive catalysis specifically targets the prediction of the performance and behaviour of catalysts. Ultimately, the goal is to identify new catalysts with optimal properties, leading to chemical processes that are both more efficient and sustainable. In this framework, %VBur can be a key metric for deepening our understanding of catalysis, emphasizing predictive catalysis and sustainability. Those latter concepts are needed to direct our efforts toward identifying the optimal catalyst for any reaction, minimizing waste, and reducing experimental efforts while maximizing the efficacy of the computational methods.

Merging electrocatalytic alcohol oxidation with C-N bond formation by electrifying metal-ligand cooperative catalysts

Electrification of thermal chemical processes could play an important role in creating a more energy efficient chemical sector. Here we demonstrate that a range of MLC catalysts can be successfully electrified and used for imine formation from alcohol precursors, thus demonstrating the first example of molecular electrocatalytic C-N bond formation.This novel concept allowed energy efficiency to be increased by an order of magnitude compared to thermal catalysis. Molecular EAO and the electrification of homogeneous catalysts can thus contribute to current efforts for the electrocatalytic generation of C-N bonds from simple building blocks.

Structures of nickel chloride and thiolate complexes supported by PCN and POCOP pincer ligands and catalytic reactivity of the chloride complexes

Nickel chloride and thiolate complexes supported by benzene-pyridine-based nonsymmetrical PCN pincer ligands, [2-(tBu2PO)-6-(2-pyrindinyl-4-R)-C6H3]NiX (R = H, CH3, CF3; X = Cl, SH, SPh), were synthesized and fully characterized. The structures of these complexes and the catalytic reactivity of the chloride complexes were investigated along with the related POCOP counterparts [2,6-(tBu2PO)2C6H3]NiX (X = Cl, SH). It was found that the composition and substitution of the pincer backbone evidently influence the structures and catalytic reactivity. The Ni-P and Ni-Cipso bond lengths in the PCN complexes are significantly shorter than those in the POCOP complex. The Ni-Cl and Ni-S bond lengths in the PCN complexes are longer than those in the POCOP complexes. An electron rich pyrindinyl ring in the PCN complexes makes the Ni-Cl bond longer. The Ni-N bond length is more sensitive to the auxiliary ligand compared with the Ni-P bond length in the PCN complexes. The PCN chloride complexes were found to be active catalysts for selective hydration of nitriles to primary amides in the presence of NaOH at 80 °C and the catalytic activity increases with the increase of electron richness of the pyridinyl ring. However, the corresponding POCOP counterpart is inactive under the same conditions.

A switchable route for selective transformation of ethylene glycol to hydrogen and glycolic acid using a bifunctional ruthenium catalyst

The developed bifunctional NNN-Ru complex features a high catalytic efficiency for the selective production of hydrogen and glycolic acid from ethylene glycol under mild reaction conditions, where a TON of 6395 was achieved. Tuning the reaction conditions afforded further dehydrogenation of the organic substrate with higher hydrogen production, and a higher TON of 25 225 was attained. The scale-up reaction yielded 1230 mL of pure hydrogen gas under the optimized reaction conditions. The role of the bifunctional catalyst was studied and mechanistic investigations were performed.

Promoting Electrocatalytic Hydrogenation of Oxalic Acid to Glycolic Acid via an Al3+ Ion Adsorption Strategy Coupled with Ethylene Glycol Oxidation

Electrocatalytic hydrogenation (ECH) of oxalic acid (OX) to produce glycolic acid (GA), an important building block of biodegradable polymers as well as application in various branches of chemistry, has attracted extensive attention in the industry, while it still encounters challenges of low reaction rate and selectivity. Herein, we reported a cation adsorption strategy to realize the efficient ECH of OX to GA by adsorbing Al³⁺ ions on an anatase titanium dioxide (TiO₂) nanosheet array, achieving 2-fold enhanced GA productivity (1.3 vs 0.65 mmol cm^-2 h^-1) with higher Faradaic efficiency (FE) (85 vs 69%) at -0.74 V vs RHE. We reveal that the Al³⁺ adatoms on TiO₂ both act as electrophilic adsorption sites to enhance the carbonyl (C═O) adsorption of OX and glyoxylic acid (intermediate) and also promote the generation of reactive hydrogen (H*) on TiO₂, thus promoting the reaction rate. This strategy is demonstrated effective for different carboxylic acids. Furthermore, we realized the coproduction of GA at the bipolar of a H-type cell by pairing ECH of OX (at cathode) and electrooxidation of ethylene glycol (at anode), demonstrating an economical manner with maximum electron economy.

Electrification of a Milstein-type catalyst for alcohol reformation

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium catalyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens a pathway to use alcohols as a renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

One pot tandem dehydrogenative cross-coupling of primary and secondary alcohols by ruthenium amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene complexes

One-pot tandem dehydrogenative cross-coupling of primary and secondary alcohols was catalyzed by three ruthenium complexes [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene]Ru(p-cymene)Cl [R = Et (1b), i-Pr (2b), Bn (3b)], of amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene (NHC) ligands. Density Functional Theory (DFT) calculations were employed for the ruthenium (1b) precatalyst to understand this reaction mechanism completely, and the mechanisms adapted are divided categorically into three steps (i) nucleophilic substitution of chloride ions by alcohols, (ii) dehydrogenation of primary and secondary alcohols, and (iii) olefin and ketone hydrogenation. Our mechanistic study reveals that the formation of a deprotonated Ru-alcoholate (A) or (E) intermediate is favorable compared to the protonated form (A') or (E') from (1b) by associative nucleophilic substitution. Though an ionic pathway that proceeds through (A') or (E'), has less barriers in the dehydrogenation and olefin/ketone hydrogenation steps than that of the neutral pathway, proceeding through (A) or (E), a steep energy barrier was observed in the first nucleophilic substitution step, prohibiting the reaction to proceed via the intermediate (A') or (E'). Thus, our thorough mechanistic study reveals that the reaction proceeds via deprotonated Ru-alcoholate (A) or (E) species. Furthermore, the 1,4 addition of an α,β-unsaturated carbonyl compound is kinetically and thermodynamically favorable over the 1,2 addition, and the experiments support these observations. As a testimony towards practical application in synthesizing bio-active flavonoid based natural products, five different flavan derivatives (16-20), were synthesized by the dehydrogenative coupling reaction using the neutral ruthenium (1-3)b complexes.

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.

Ruthenium-Catalyzed Dehydrogenative Functionalization of Alcohols to Pyrroles: A Comparison between Metal-Ligand Cooperative and Non-cooperative Approaches

Herein, we report the synthesis and characterization of two ruthenium-based pincer-type catalysts, [1]X (X = Cl, PF₆) and 2, containing two different tridentate pincer ligands, 2-pyrazolyl-(1,10-phenanthroline) (L¹) and 2-arylazo-(1,10-phenanthroline) (L^2a/2b, L^2a = 2-(phenyldiazenyl)-1,10-phenanthroline; L^2b = 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline), and their application in the synthesis of substituted pyrroles via dehydrogenative alcohol functionalization reactions. In catalyst [1]X (X = Cl, PF₆), the tridentate scaffold 2-pyrazolyl-(1,10-phenanthroline) (L¹) is apparently redox innocent, and all the redox events occur at the metal center, and the coordinated ligands remain as spectators. In contrast, in catalysts 2a and 2b, the coordinated azo-aromatic scaffolds are highly redox-active and known to participate actively during the dehydrogenation of alcohols. A comparison between the catalytic activities of these two catalysts was made, starting from the simple dehydrogenation of alcohols to further dehydrogenative functionalization of alcohols to various substituted pyrroles to understand the advantages/disadvantages of the metal-ligand cooperative approach. Various substituted pyrroles were prepared via dehydrogenative coupling of secondary alcohols and amino alcohols, and the N-substituted pyrroles were synthesized via dehydrogenative coupling of aromatic amines with cis-2-butene-1,4-diol and 2-butyne-1,4-diol, respectively. Several control reactions and spectroscopic experiments were performed to characterize the catalysts and establish the reaction mechanism.

Synthesis of α-Hydroxy Acids via Dehydrogenative Cross-Coupling of a Sustainable C2 Chemical (Ethylene Glycol) with Alcohols

Ir(NHC) (NHC, N-heterocyclic carbene)-catalyzed dehydrogenative coupling of sustainable ethylene glycol and various bioalcohols can produce industrially valuable α-hydroxy acids (AHAs). This study is the first to report the sustainable synthesis of higher C_n AHAs, in addition to glycolic acid (C₂ AHA) and lactic acid (C₃ AHA). This catalytic system can be recycled to the seventh cycle while maintaining good yields. A reaction mechanism, including facile dehydrogenation of each alcohol and fast cross-coupling of dehydrogenated aldehydes forming products, was proposed based on ¹⁸O- and ²H-labeling experiments and electron spray ionization-mass spectrometry (ESI-MS) and NMR spectral analyses.

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.

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.