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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]

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]

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]

This study adapts the biphasic OrganoCat system into a flow-through (FT) reactor, using a heated tubular setup where a mixture of oxalic acid and 2-methyltetrahydrofuran (2-MTHF) is pumped through beech wood biomass. This method minimizes solvent-biomass contact time, facilitating rapid product removal and reducing the risk of secondary reactions. A comparative analysis with traditional batch processes reveals that the FT system, especially under severe conditions, significantly enhances extraction efficiency, yielding higher amounts of lignin and sugars with reduced solid residue. Notably, the FT system shows partial hydrolysis of the cellulose, which increases with temperature while not producing significant amounts of furfural or 5-HMF, indicating more efficient depolymerization of polysaccharides without substantial sugar degradation. A statistical design of experiments (DOE) using a Box-Behnken design elucidates the influence of process variables (time, solvent flow rate, temperature) on the yield. Key findings highlight reactor temperature as the dominant factor affecting yields, with process time showing a significant but less pronounced impact. This study demonstrates the potential of the FT OrganoCat system for efficient lignocellulosic biomass fractionation and represents an advancement towards continuous lignocellulose processing, contributing to our knowledge of process optimization for improved biorefinery applications., (© 2024 Wiley‐VCH GmbH.)

An adaptive catalytic system for selective hydrogenation was developed exploiting the H 2  + CO 2   ⇔  HCOOH equilibrium for reversible, rapid, and robust on/off switch of the ketone hydrogenation activity of ruthenium nanoparticles (Ru NPs). The catalyst design was based on mechanistic studies and DFT calculations demonstrating that adsorption of formic acid to Ru NPs on silica results in surface formate species that prevent C═O hydrogenation. Ru NPs were immobilized on readily accessible silica supports modified with guanidinium-based ionic liquid phases (Ru@SILP GB ) to generate in situ sufficient amounts of HCOOH when CO 2 was introduced into the H 2 feed gas for switching off ketone hydrogenation while maintaining the activity for hydrogenation of olefinic and aromatic C═C bonds. Upon shutting down the CO 2 supply, the C═O hydrogenation activity was restored in real time due to the rapid decarboxylation of the surface formate species without the need for any changes in the reaction conditions. Thus, the newly developed Ru@SILP GB catalysts allow controlled and alternating production of either saturated alcohols or ketones from unsaturated substrates depending on the use of H 2 or H 2 /CO 2 as feed gas. The major prerequisite for design of adaptive catalytic systems based on CO 2 as trigger is the ability to shift the H 2  + CO 2   ⇔  HCOOH equilibrium sufficiently to exploit competing adsorption of surface formate and targeted functional groups. Thus, the concept can be expected to be more generally applicable beyond ruthenium as the active metal, paving the way for next-generation adaptive catalytic systems in hydrogenation reactions more broadly.

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 ]NTf 2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the Rh I parents, Rh -I d 10 -complexes were obtained and characterized spectroscopically, including 103 Rh 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 E 0 (Rh I/0 ) and second E 0 (Rh 0/-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., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The reaction with in situ extraction to yield 5-hydroxymethylfurfural (HMF) from d-fructose (Fru) was investigated using a biphasic system based on a self-consuming deep eutectic solvent (DES) as reaction phase. The significance of choline chloride (ChCl), a cost-effective and safe quaternary ammonium salt, was evident in enhancing HMF yield through fructose dehydration and concurrently suppressing side reactions. The DES system demonstrated fast reactions with high selecivities and recyclability across five cycles. The observed decline in H 4 SiW 12 O 40 activity, primarily due to proton leaching, was successfully restored with the addition of HCl. Furthermore, ChCl exhibited ease of recrystallization in the presence of acetonitrile. This research proposes an environmentally friendlier approach for HMF production through a reusable-biphasic process. The presented reaction system suppresses completely the formation of levulinic and formic acid leading to HMF yields of up to 84 % of selectivities of up to 88 % after 30 minutes at 80 °C. The system was recycled over 16 runs and after an initial slight loss of activity the system in the run 0-5, run 6-15 has shown a constant HMF output as in the first recycling run., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N 2 with H 2 , leading to NH 3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M 1 (H) 2 ] (M 1 = W/Mo) were chosen to bind N 2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M 2 {2-(2-pyridyl)phenyl}(CH 3 CN)] + (Cp* = C 5 (CH 3 ) 5 and M 2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H 2 splitting. The stepwise transfer of protons and hydrides to the bound N 2 and intermediate N x H y units results in the formation of NH 3 . 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 N 2 /H 2 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.

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 MnO x @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, MnO x @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., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Manganese(I) carbonyl complexes bearing a MACHO-type ligand (HN(CH 2 CH 2 PR 2 ) 2 ) readily react in their amido form with CO 2 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 CO 2 -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., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)