The novel [Ru(Acriphos)(PPh₃)(Cl)(PhCO₂)] [1; Acriphos=4,5-bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO₂ to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H₂O without the need for amine bases as co-reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h⁻¹ were achieved, thus rendering 1 one of the most active catalysts for CO₂ hydrogenations under additive-free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L⁻¹) was shown to be limited by product-dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L⁻¹, which corresponds to optimized values of TON=16×10³ and TOF_avg≈10³ h⁻¹.

The novel [Ru(Acriphos)(PPh₃)(Cl)(PhCO₂)] [1; Acriphos=4,5-bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO₂ to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H₂O without the need for amine bases as co-reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h⁻¹ were achieved, thus rendering 1 one of the most active catalysts for CO₂ hydrogenations under additive-free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L⁻¹) was shown to be limited by product-dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L⁻¹, which corresponds to optimized values of TON=16×10³ and TOF_avg≈10³ h⁻¹.

Phosphoramidite ligands based on pinene-derived chiral amines have been prepared by a straightforward procedure in good yields. The key step of the synthetic protocol is a stereoselective hydrogenation of annulated pinene–pyridine derivatives leading to (diastereoisomeric) secondary amines that were separated and treated with different chlorophosphites to yield the envisaged phosphoramidites. The absolute configurations of the ligands were assigned on the basis of NMR analyses and corroborated by X-ray diffraction analysis of a borane adduct of a typical ligand. The new ligands were employed in the asymmetric hydrogenation of imines and olefins. The iridium-catalyzed hydrogenation of imines provided up to 81 % ee, whereas in the rhodium-catalyzed hydrogenation of functionalized olefins enantioselectivities of up to 99 % ee were achieved. In this particular application, the different chiral elements of the ligand structure led to synergistic effects and the enantioselectivity is dominated by the chiral diol moiety.

Upon the simple addition of substrates, the ruthenium pincer complex [Ru(^tBuPNP)(H₂)(H)₂] [1; ^tBuPNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] is an active and selective catalyst system for the hydrosilylation of terminal alkyl alkynes under mild, solvent-free conditions. The reactivity of this system for other functionalized terminal alkynes was also investigated, and we observed competing catalytic cycles that produce both alkyne dimers and dehydrogenative silylation products. Kinetic measurements for the hydrosilylation of 1-octyne show that the catalyst has an initial turnover frequency of 121 h^–1 at room temperature. The stoichiometric reaction between 1 and H₂SiPh₂ yields [Ru(^tBuPNP)(H)₂(H₂SiPh₂)], which undergoes Si–H bond activation to yield the catalytically active species [Ru(^tBuPNP)(HSiPh₂)(H)]. The reaction of 1 with phenylacetylene yielded [Ru(^tBuPNP)(H)₂(HC≡CPh)] and [Ru(^tBuPNP)(H)(C≡CPh)(HC≡CPh)], and we propose that the latter is the active species in the dimerization reaction.

Ruthenium nanoparticles immobilized on acid-functionalized supported ionic liquid phases (Ru NPs@SILPs) act as efficient bifunctional catalysts in the hydrodeoxygenation of phenolic substrates under batch and continuous flow conditions. A synergistic interaction between the metal sites and acid groups within the bifunctional catalyst leads to enhanced catalytic activities for the overall transformation as compared to the individual steps catalyzed by the separate catalytic functionalities.

A library of α,α,α,α-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL)-based phosphoramidites has been synthesized and applied in the Ni-catalyzed cycloisomerization of different dienes. Through the systematic variation of the three structural motifs of the lead structure, that is, the amine moiety, the protecting group, and the aryl substituents, the ligand features could be optimized for the asymmetric cycloisomerization of the model substrate diethyl diallylmalonate. The substrate scope of the new catalytic system was extended to other diallylic substrates, including unsymmetrical dienes. Overall remarkably high activities of up to approximately 13 500 h⁻¹, very high selectivities toward five-membered exo-methylenecyclopentanes, and enantioselectivities of up to 92 % ee have been achieved.

Ruthenium nanoparticles immobilized on acid-functionalized supported ionic liquid phases (Ru NPs@SILPs) act as efficient bifunctional catalysts in the hydrodeoxygenation of phenolic substrates under batch and continuous flow conditions. A synergistic interaction between the metal sites and acid groups within the bifunctional catalyst leads to enhanced catalytic activities for the overall transformation as compared to the individual steps catalyzed by the separate catalytic functionalities.

A continuous-flow process for the asymmetric hydrogenation of methyl propionylacetate as a prototypical β-keto ester in a biphasic system of ionic liquid and supercritical carbon dioxide (scCO₂) is presented. An established ruthenium/2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) catalyst was immobilised in an imidazolium-based ionic liquid while scCO₂ was used as mobile phase transporting reactants in and products out of the reactor. The use of acidic additives led to significantly higher reaction rates and enhanced catalyst stability albeit at slightly reduced enantioselectivity. High single pass conversions (>90%) and good enantioselectivity (80–82% ee) were achieved in the first 80 h. The initial catalyst activity was retained to 91% after 100 h and to 69% after 150 h time-on-stream, whereas the enantioselectivity remained practically constant during the entire process. A total turnover number of ∼21,000 and an averaged space-time yield (STY_av) of 149 g L⁻¹ h⁻¹ were reached in a long-term experiment. No ruthenium and phosphorus contaminants could be detected via inductively coupled plasma optical emission spectrometry (ICP-OES) in the product stream and almost quantitative retention by the analysis of the stationary phase was confirmed. A comparison between batch-wise and continuous-flow operation on the basis of these data is provided.

The structures of three ruthenium pincer polyhydride complexes, [Ru(PCP)(H)(H₂)_n] {PCP = 2,5-bis(di-tert-butylphosphanylmethyl)benzene; n = 2 (1), n = 1 (2)} and [Ru(PNP)(H)₂(H₂)] {PNP = 2,5-bis(di-tert-butylphosphanyl)lutidine (3)}, were investigated by X-ray and neutron diffraction for 1 and only X-ray diffraction for 3, whereas all of the compounds together with the dynamics of the hydrogen centers were investigated by DFT calculations. All three analytical methods together generate a detailed understanding of the structures. DFT calculations, in particular, explain the occurrence of 1 and 2 in THF solution as well as the dynamics of the movement of the hydrogen centers in 3.

Very few cases of oxidative addition of NH₃ to transition-metal complexes forming terminal amide hydrides have been experimentally observed. Here, two examples with the iridium pincer complexes [Ir(PCP)(NH₃)] A1 with PCP=[κ³-(tBu₂P-C₂H₄)₂CH]⁻ and [Ir(PSiP)(NH₃)] B1 with PSiP=[κ³-(2-Cy₂P-C₆H₄)₂SiMe]⁻ were investigated by DFT calculations applying the M06L density functional to successfully reproduce the trend of the experimentally observed thermochemical stabilities. According to the calculations, the corresponding hydrido-amido complexes A2 and B2 are more stable than the corresponding ammine complexes by ΔG=−2.8 and −2.6 kcal mol⁻¹, respectively. Complexes such as A2 and B2 are ideally suited entry points to catalytic cycles for the hydroamination of ethylene with ammonia. Therefore, the relevant stationary points of the potentially available cycles were studied computationally to verify if these complexes can catalyze the hydroamination. As a result, complex A2 will clearly not catalyze the hydroamination as all energy spans calculated range close to 40 kcal mol⁻¹ or higher. The energy spans obtained with B2 are significantly lower in some cases and range around 35 kcal mol⁻¹, further indicating that no turnover can be expected. By systematically varying the structure of B2, the energy span could be reduced to 28.8 kcal mol⁻¹ corresponding to a TOF of 17 h⁻¹ at a reaction temperature of 140 °C. A reoptimization of relevant structures under the inclusion of cyclohexane as a typical solvent reduces the calculated TOF to 6.0 h⁻¹.

A continuous-flow process based on a chiral transition-metal complex in a supported ionic liquid phase (SILP) with supercritical carbon dioxide (scCO₂) as the mobile phase is presented for asymmetric catalytic transformations of low-volatility organic substrates at mild reaction temperatures. Enantioselectivity of >99 % ee and quantitative conversion were achieved in the hydrogenation of dimethylitaconate for up to 30 h, reaching turnover numbers beyond 100 000 for the chiral QUINAPHOS–rhodium complex. By using an automated high-pressure continuous-flow setup, the product was isolated in analytically pure form without the use of any organic co-solvent and with no detectable catalyst leaching. Phase-behaviour studies and high-pressure NMR spectroscopy assisted the localisation of optimum process parameters by quantification of substrate partitioning between the IL and scCO₂. Fundamental insight into the molecular interactions of the metal complex, ionic liquid and the surface of the support in working SILP catalyst materials was gained by means of systematic variations, spectroscopic studies and labelling experiments. In concert, the obtained results provided a rationale for avoiding progressive long-term deactivation. The optimised system reached stable selectivities and productivities that correspond to 0.7 kg L⁻¹ h⁻¹ space–time yield and at least 100 kg product per gram of rhodium, thus making such processes attractive for larger-scale application.

One of the major challenges for the utilization of carbon dioxide as a chemical feedstock is to devise thermodynamically feasible transformations to valuable chemicals through efficient catalytic processes. In this paper, we examine the feasibility of the experimentally not yet realized direct hydrocarboxylation of alkenes, using computational methods. A conceivable catalytic cycle was devised for the addition of H₂ and CO₂ to ethene, which affords propionic acid. The corresponding energy profiles were calculated with state-of-the-art DFT methods for three rhodium pincer complexes as potential catalyst. Several junctions within the productive catalytic cycle were identified, leading to competing hydrogenation reactions of ethene or CO₂. A profound analysis of the reaction network by means of the energetic span model allowed us to identify parameters that facilitate the carboxylation and that favor it over the hydrogenation reaction. A comparison of the relevant activation energies revealed that two of the three investigated complexes slightly favour the hydrogenation of ethene, but one complex preferentially stays within the carboxylation cycle.

A prototypical catalytic cycle for the direct carboxylation of unactivated arene CH bonds with CO₂ based on ruthenium(II) pincer complexes as catalysts is proposed and investigated by density functional theory (DFT) methods. The energetic span model is used to predict the turnover frequency (TOF) of various potential catalysts, evaluating their efficiency for this reaction. In addition to modifications of the catalyst structure, we also investigated the effect of the substrate, the solvent, and the influence of a base on the thermodynamics and kinetics of the reaction. Turnover frequencies in the range of 10⁵–10⁷ h⁻¹ are predicted for the best systems. Alternative reaction pathways that might prevent the reaction are also investigated. In all cases, either the respective intermediates are found to be unstable or activation barriers are found to be very high, thereby indicating that these alternative pathways will not interfere with the proposed catalytic cycle. As a result, several ruthenium pincer complexes are suggested as very promising candidates for experimental investigation as catalysts for the carboxylation of arene CH bonds with CO₂.

Catalytic hydrogenation of nitriles to amines by nonclassical ruthenium hydride complexes derived from PNP pincer ligands is described. Aromatic as well as aliphatic nitriles are reduced to the corresponding primary amines. Hydrogen pressure influences the selectivity for the primary amines. The mechanism of nitrile reduction with nonclassical ruthenium hydride pincer complexes is investigated by DFT calculations. A catalytic cycle involving the coordination of nitrile trans to the pincer backbone after an initial hydride rearrangement at the ruthenium center, and the subsequent first transfer of the hydride ligand to the carbon center of the nitrile ligand is suggested as a possible reaction mechanism. Interestingly, the use of water as additive increases the selectivity for the primary amines and the rate of the reactions.

While experts in various fields discuss the potential of carbon capture and storage (CCS) technologies, the utilization of carbon dioxide as chemical feedstock is also attracting renewed and rapidly growing interest. These approaches do not compete; rather, they are complementary: CCS aims to capture and store huge quantities of carbon dioxide, while the chemical exploitation of carbon dioxide aims to generate value and develop better and more-efficient processes from a limited part of the waste stream. Provided that the overall carbon footprint for the carbon dioxide-based process chain is competitive with conventional chemical production and that the reaction with the carbon dioxide molecule is enabled by the use of appropriate catalysts, carbon dioxide can be a promising carbon source with practically unlimited availability for a range of industrially relevant products. In addition, it can be used as a versatile processing fluid based on its remarkable physicochemical properties.

Catalytic carboxylation reactions that use CO₂ as a C1 building block are still among the ‘dream reactions’ of molecular catalysis. To obtain a deeper insight into the factors that control the fundamental steps of potential catalytic cycles, we performed a detailed computational study of the insertion reaction of CO₂ into rhodium–alkyl bonds. The minima and transition-state geometries for 38 pincer-type complexes were characterized and the according energies for the CC bond-forming step were determined. The electronic properties of the Rhalkyl bond were found to be more important for the magnitude of the activation barrier than the interaction between rhodium and CO₂. The charge of the alkyl-chain carbon atom, as well as agostic and orbital interactions with the rhodium, exhibit the most pronounced influence on the energy of the transition states for the CO₂ insertion reaction. By varying the backbone and the donor groups of the pincer ligand those properties can be tuned over a very broad range. Thus, it is possible to match the electronic and steric properties with the fundamental requirements of the CO₂ insertion into rhodium–alkyl bonds of the ligand framework.

The catalytic hydroamination of ethylene with ammonia was investigated by means of density functional theory (DFT) calculations. An initial computational screening of key reaction steps (CN bond formation, NH bond cleavage), which are assumed to be part of a catalytic cycle, was carried out for complexes with the [M(L)]-complex fragment (M=Rh, Ir; L=NCN, PCP; NCN=2,5-bis(dimethylaminomethyl)benzene, PCP=2,5-bis- (dimethylphosphanylmethyl)benzene). Based on the evaluation of activation barriers, this screening showed the rhodium compound with the NCN ligand to be the most promising catalyst system. A detailed investigation was carried out starting with the hypothetical catalyst precursor [Rh(NCN)(H)₂(H₂)] (1). A variety of activation pathways to yield the catalytically active species [Rh(NCN)(H)(NH₂)] (5), as well as [Rh(NCN)(C₂H₅)(NH₂)] (17), were identified. With 5 and 17 several closed catalytic cycles could be calculated. One of the calculated cycles is favoured kinetically and bond-forming events have activation barriers low enough to be put into practice. The calculations also show that for experimental realisation the synthesis of 1 is not necessary, as the synthesis of 17 would establish an active catalyst directly without the need for activation. Oligomerisation of ethylene would be possible in principle and would be expected as a competitive side reaction. Accordingly not only ethylamine would be observed in an experimental system, as amines with longer carbon chains also can be formed.

New derivatives of the Quinaphos ligands and the related Dihydro-Quinaphos ligands based on the more flexible 1,2,3,4-tetrahydroquinoline backbone have been prepared and fully characterised. A general and straightforward separation protocol was devised, which allowed for the gram-scale isolation of the R_a,S_c and S_a,R_c diastereomers. These new phosphine–phosphoramidite ligands have been applied in the Rh-catalysed asymmetric hydrogenation of functionalised olefins with the achievement of excellent enantioselectivities (≥99 %) in most cases and turnover frequency (TOF) values of up to ≥20 000 h⁻¹. These results substantiate the practical utility of readily accessible Quinaphos-type ligands, which belong to the most active and selective category of ligands for Rh-catalysed hydrogenation known to date.

At room temperature, nickel catalysts based on the new phosphoramidite (11bR)-N-[(S)-1-(naphthalen-1-yl)ethyl]-N-[(S)-1-(naphthalen-2-yl)ethyl]dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepin-4-amine provide excellent selectivities for 3-arylbut-1-enes (93–99%) with high enantioselectivities (90–95% ee) and TOFs (up to 8300 h⁻¹) in the hydrovinylation of electron-rich and electron-poor vinylarenes. Within a few minutes, useful chiral building blocks and intermediates can be synthesized using this practical catalytic system.

A set of chiral monodentate phosphorous triamides (PTA) comprising 1,1′-binaphthyl-2,2′-diamine as the common moiety have been synthesised. Electronic and steric tuning of the ligands was achieved by variation of the substituents at the diamine nitrogen atoms by incorporating methyl, p-tolyl and tosyl groups. Both chiral and achiral building blocks were used as monoamine components. Notably, (11bR)-3,5-dimethyl-N,N-bis[(S)-1-phenylethyl]-3,5-dihydro-4H-dinaphtho[2,1-d:1′,2′-f][1,3,2]diazaphosphepin-4-amine, which is the PTA most closely related to the Feringa phosphoramidite ligand, was synthesised and characterised by NMR spectroscopy and X-ray diffraction. This PTA displays increased conformational rigidity in comparison with the corresponding phosphoramidite and adopts a C₁-symmetric structure both in the solid state and in solution, even at room temperature. The new PTA ligands were used in the copper-catalysed conjugate addition of diethylzinc to cyclohex-2-enone and the nickel-catalysed hydrovinylation of styrene giving good activities and chemoselectivities at moderate enantioselectivities in both reactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A broad range of commercially available Lewis acids were investigated for their ability to activate and regulate nickel catalysts for asymmetric hydrovinylation processes using styrene as model substrate and ligand (R_a,S_C,S_C)-5 as benchmark system. The colour change during the activation step associated with the halide abstraction furnishes helpful indications to adapt the activation conditions to the pre-catalyst/Lewis acid system. In general, metal halide Lewis acids led to higher activities and enantioselectivities than the corresponding triflates. In particular, the use of InI₃ as co-catalyst resulted in the same chemo- and enantioselectivity and even higher activity than the benchmark system based onNaBArF. Moreover, InI₃ is safe to handle and cheap, thus providing a simple and practical protocol for an efficient hydrovinylation reaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Highly stereoselective cycloisomerization of 1,6-dienes with a catalyst system based on [Ni(allyl)(cod)][BARF] and Wilke’s azaphospholene all-(R)-1 is described. Unprecedented high enantiomeric excesses of up to 91% were obtained for the formation of five-membered cyclic products from symmetrically substituted 1,6-dienes. High chemo- and regioselectivities were achieved also for unsymmetrically 1,6-dienes substituted in terminal position leading again to five-membered cyclic products in fair to very good yields. Cycloisomerization products were subjected to sequential hydroboration as well as hydrogenation of the exo-methylene functional group. Promising diastereoselectivities were achieved and these reaction sequences could be efficiently performed in a one-pot procedure.

Catalytic H/D-exchange reactions were studied with [Ru(dtbpmp)(η²-H₂)(H)₂] (1) as catalyst. Under mild reaction conditions (25–75 °C) a wide range of arenes and olefins undergo H/D exchange with [D₆]benzene. A preference for protons at sp² carbons was observed with conversions up to >90 % and significant regioselectivity in certain cases. For more reaction insights NMR-based kinetic studies were performed with naphthalene as substrate, revealing an activation energy of 15.8 kcal mol^–1 for the H/D exchange at the β-position. Furthermore, the key steps of the reaction mechanism were investigated by means of DFT calculations for both model complexes (PMe₂ donor sites) and real catalysts (PtBu₂ donor sites). The calculations resulted in Gibb's free activation energies in the range of 10–16 kcal mol^–1, indicating H/D exchange at the β-position of naphthalene to be clearly favoured over the α-position, which is in full accordance with the experimental observations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)