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Hydrogen as energy vector from renewable sources and carbon dioxide as carbon source are central elements of a future sustainable interface between energy and chemistry. While often viewed merely as "substitutes" for fossil resources, the current article discusses opportunities to open new synthetic pathways and to generate novel molecular architectures for the delivery of the same or even improved functionalities expected from chemical products. Catalysis is the key science and technology in this endeavour and three general principles for the desing of catalytic systems are proposed as guidelines for fundamental research. This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'. [ABSTRACT FROM AUTHOR]

Rhodium complexes in the −I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two‐electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P‐substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh−Id10‐complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/−I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P−Rh‐P bite angles provide only partial correlations with the redox potentials. However, we identified the C−O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two‐electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions. [ABSTRACT FROM AUTHOR]

A simple and recyclable homogeneous catalytic system for the hydrogenation of carbon monoxide to methanol was established. The reaction is catalyzed by a molecular manganese complex using a high-boiling alcohol as the solvent for catalyst immobilization. The CO hydrogenation is assisted by the product itself and the solvent through the formation of a methyl or dodecyl formate ester intermediate mediated by catalytic amounts of NaOMe as the base. This allows the catalytic formation of methanol in alcohols combined with facile product separation and catalyst recycling via distillation. Initial turnover frequencies (TOF) of 2250 h−1 were reached under optimized conditions in 1-dodecanol/methanol as the reaction medium (T = 160 °C, p(H2/CO) = 80/10 bar). The performance was stabilized in batch-wise recycling over 6 runs achieving a total turnover number (TTON) of >12 000 corresponding to an enhancement of more than five times compared to single batch operation under identical conditions. Minimal leaching of the components of the organometallic catalyst was observed during distillative product separation and catalyst activity could be fully restored by re-addition of the base NaOMe. [ABSTRACT FROM AUTHOR]

This work focuses on the development and investigation of a Rh/TPP catalytic system for the synthesis of saturated aliphatic carboxylic acids from readily available oxygenated substrates, H2 and non-toxic, renewable CO2. Optimization of the reaction conditions and reagents was carried out using 2-butanol and 1-butanol as typical secondary and primary alcohols. Afterwards, the reaction system was investigated in-depth with a separate investigation of the reverse water–gas-shift-reaction and the carbonylation reaction as key steps of the overall tandem transformation. Based on these results, a reaction network was established with two distinct main pathways. While the secondary alcohols are converted preferentially via acid catalysed dehydration to the corresponding olefins followed by hydroxycarbonylation, the primary alcohols react primarily via nucleophilic substitution to the iodide compound followed by a Monsanto-type carbonylation. Based on these results, a broad range of alcohols including bio-based substrates was converted to the corresponding C1 elongated carboxylic acids. Additionally, other oxygenated substrates such as aldehydes, ketones and industrially relevant substrate mixtures were applied successfully. [ABSTRACT FROM AUTHOR]

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol−1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol−1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected. [ABSTRACT FROM AUTHOR]

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol−1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol−1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected. [ABSTRACT FROM AUTHOR]

Colloidal and supported manganese nanoparticles were synthesized following an organometallic approach and applied in the catalytic transfer hydrogenation (CTH) of aldehydes and ketones. Reaction parameters for the preparation of colloidal nanoparticles (NPs) were optimized to yield small (2–2.5 nm) and well‐dispersed NPs. Manganese NPs were further immobilized on an imidazolium‐based supported ionic phase (SILP) and characterized to evaluate NP size, metal loading, and oxidation states. Oxidation of the Mn NPs by the support was observed resulting in an average formal oxidation state of +2.5. The MnOx@SILP material showed promising performance in the CTH of aldehydes and ketones using 2‐propanol as a hydrogen donor, outperforming previously reported Mn NPs‐based CTH catalysts in terms of metal loading‐normalized turnover numbers. Interestingly, MnOx@SILP were found to lose activity upon air exposure, which correlates with an additional increase in the average oxidation state of Mn as revealed by X‐ray absorption spectroscopic studies. [ABSTRACT FROM AUTHOR]

Manganese(I) carbonyl complexes bearing a MACHO‐type ligand (HN(CH2CH2PR2)2) readily react in their amido form with CO2 to generate 4‐membered {Mn−N−C−O} metallacycles. The stability of the adducts decreases with the steric demand of the R groups at phosphorous (R=isopropyl>adamantyl). The CO2‐adducts display generally a lower reactivity as compared to the parent amido complexes. These adducts can thus be interpretated as masked forms of the active amido catalysts and potentially play important roles as off‐loop species or branching points in catalytic transformations of carbon dioxide. [ABSTRACT FROM AUTHOR]

Ruthenium nanoparticles (NPs) immobilized on an amine‐functionalized polymer‐grafted silica support act as adaptive catalysts for the hydrogenation of bicyclic heteroaromatics. Whereas full hydrogenation of benzofuran and quinoline derivatives is achieved under pure H2, introducing CO2 into the H2 gas phase leads to an effective shutdown of the arene hydrogenation while preserving the activity for the hydrogenation of the heteroaromatic part. The selectivity switch originates from the generation of ammonium formate species on the surface of the materials by catalytic hydrogenation of CO2. The CO2 hydrogenation is fully reversible, resulting in a robust and rapid switch between the two states of the catalyst adapting its performance in response to the feed gas composition. A variety of benzofuran and quinoline derivatives were hydrogenated to fully or partially saturated products in high selectivity and yields simply by altering the composition of the feed gas from H2 to H2/CO2. The adaptive catalytic system thus provides controlled access to valuable products using a single catalyst rather than two specific and distinct catalysts with static reactivity. [ABSTRACT FROM AUTHOR]

Iron–platinum nanoparticles of tunable Fe/Pt ratios were immobilized on an imidazolium-based supported ionic liquid phase (SILP). The resulting FexPt100–x@SILP materials were characterized and applied in catalysis. Electron microscopy confirmed the formation of small, well-dispersed, bimetallic nanoparticles in all the investigated materials. Magnetometry measurements evidenced a clear influence of the Fe/Pt ratio on the oxidation state of Fe in FexPt100–x@SILP. In particular, Fe appeared in the metallic state at x ≤ 40, whereas partial oxidation was observed at x ≥ 60. The catalytic properties of FexPt100–x@SILP materials were probed using the reduction of benzylic ketones with H2 as a model reaction. The Fe/Pt ratio of the nanoparticles was found to have a critical influence on the observed activity and selectivity, with clear synergistic effects. In particular, by diluting Pt with Fe sites, the ability of FexPt100–x@SILP catalysts to hydrogenate 6-membered aromatic rings was shut down for x ≥ 40, while CO hydrogenation activity was preserved. In addition, Fe40Pt60@SILP showed excellent hydrodeoxygenation activity and selectivity with acetophenone derivatives in the absence of acid additives. This remarkable reactivity, out of reach for monometallic Pt@SILP, is attributed to the enhanced oxophilicity of Fe in FePt catalysts favoring CO activation and cleavage. [ABSTRACT FROM AUTHOR]

The rapidly growing importance of green hydrogen and renewable carbon resources as essential feedstocks for sustainable chemical value chains opens room for disruptive innovations regarding chemical production processes. The fluctuation and variability associated with non‐fossil energy and raw material supply holds many challenges for catalysts to cope with the resulting dynamics. However, many new opportunities also arise once catalyst design starts to aim at performance that is "adaptive" rather than "task‐specific". In this Scientific Perspective, we propose to define adaptivity in catalysis on the basis of three essential properties that are reversibility, rapidity, and robustness (R3 rule). Promising design strategies and selected examples are described to substantiate the scientific concept and to highlight its potential for chemical energy conversion. [ABSTRACT FROM AUTHOR]

Die stark wachsende Bedeutung von grünem Wasserstoff und erneuerbaren Kohlenstoffressourcen als Rohstoffbasis für nachhaltige Wertschöpfungsketten eröffnet Raum für bahnbrechende Innovationen bei chemischen Produktionsprozessen. Die Fluktuation und Variabilität, die mit der Versorgung mit nicht‐fossilen Energieträgern und Rohstoffen einhergehen, stellen Katalysatoren vor viele Herausforderungen, um die daraus resultierende Dynamik zu bewältigen. Allerdings ergeben sich auch viele neue Möglichkeiten, wenn das Katalysatordesign auf eine Leistung abzielt, die "adaptiv" und nicht "aufgabenspezifisch" ist. In diesem Beitrag schlagen wir vor, Adaptivität in der Katalyse auf der Grundlage von drei wesentlichen Eigenschaften zu definieren, nämlich Reversibilität, Rapidität und Robustheit (R3‐Regel). Zur Begründung des wissenschaftlichen Konzepts und zur Darstellung seines Potenzials für die chemische Energiekonversion werden vielversprechende Designstrategien und ausgewählte Beispiele beschrieben. [ABSTRACT FROM AUTHOR]

One approach to recycle homogeneous catalysts is through multiphase catalysis. Multiphase catalysis is not only limited to liquid–liquid multiphase systems but also includes for example solid and liquid phases. In this work, we present a catalyst recycling system based on the crystallization of the entire catalyst phase after the reaction at ambient temperature. Using the green and polar solvent ethylene carbonate, the polar Rh/sulfoXantphos catalyst is trapped in the crystallized ethylene carbonate phase. The product can be decanted under air as the catalyst is stabilized in the solid phase and the entire solid phase including the solvent is recycled. Several reactions such as the hydroformylation of hexene, octene and decene, and the hydrogenation of C14 aldol products were conducted with this system. A TTON of 8627 could be achieved in the hydroformylation of 1-octene with initial turnover frequencies up to 1460 h−1. In addition, the catalyst "pill" was switched between different reactions to show the flexibility of the system. [ABSTRACT FROM AUTHOR]

All catalysts have unique abilities. This is especially true for microbial, enzymatic, and organometallic catalysis, which are often seen as competitive approaches preventing the exploitation of their complementarity. An increasing number of examples show, how using the complete catalytic spectrum can open roads from new substrates to new products. C1‐compounds such as formate, formaldehyde, methanol, or methane from CO2 in combination with green H2 are likely to be future sources of carbon feedstock. This short review highlights how combinations of different catalyst types can facilitate integrated reaction sequences with biogenic substrates to form "bio‐hybrid" fuels and products. [ABSTRACT FROM AUTHOR]