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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]

The potential of Pd/Pt complexes for catalytic carboxylation of arenes with CO2 is investigated by means of computational chemistry. Recently we reported that the bis[(2‐methoxyphenyl)phosphino]‐benzenesulfonamido palladium complex 1 inserts CO2 reversibly in its Pd−C(aryl) bond generating carboxylato complex 2. In the present work we study how geometric and electronic factors of various ligands and substrates influence the overall activation barrier (energy span, ES) of a potential catalytic cycle for arene carboxylation comprising this elementary step. The tendency of the key intermediates to dimerize and thus deactivating the potential catalysts is examined as well as the role of the base, which inevitably is needed to stabilize the reaction product. We show that Pd and Pt complexes I(Pd)‐L16‐S1 and I(Pt)‐L16‐S1 do not dimerize, enable the computation of complete catalytic cycles, and show interestingly low ES values of 26.8 and 24.5 kcal/mol, respectively. [ABSTRACT FROM AUTHOR]

While industrial NH3 synthesis based on the Haber-Bosch-process was invented more than a century ago, there is still no molecular catalyst available which reduces N2 in the reaction system N2/H2 to NH3. As the many efforts of experimentally working research groups to develop a molecular catalyst for NH3 synthesis from N2/H2 have led to a variety of stoichiometric reductions it seems justified to undertake the attempt of systematizing the various approaches of how the N2 molecule might be reduced to NH3 with H2 at a transition metal complex. In this contribution therefore a variety of intuition-based concepts are presented with the intention to show how the problem can be approached. While no claim for completeness is made, these concepts intend to generate a working plan for future research. Beyond this, it is suggested that these concepts should be evaluated with regard to experimental feasibility by checking barrier heights of single reaction steps and also by computation of whole catalytic cycles employing density functional theory (DFT) calculations. This serves as a tool which extends the empirically driven search process and expands it by computed insights which can be used to rationalize the various challenges which must be met. [ABSTRACT FROM AUTHOR]

A series of SN1-type reactions has been studied under various conditions to clarify the role of supercritical carbon dioxide (scCO2) as reaction medium for this kind of transformations. The application of scCO2 did not result in higher yields in any of the experiments in comparison to those under neat conditions or in the presence of other inert compressed gases. High-pressure UV/Vis spectroscopic measurements were carried out to quantify the degree of carbocation formation of a highly SN1-active alkyl halide as a function of the applied solvent. No measureable concentration of carbocations could be detected in scCO2, just like in other low polarity solvents. Taken together, these results do not support the previously claimed activating effect via enhanced SN1 ionization due to the quadrupolar moment of the supercritical fluid. [ABSTRACT FROM AUTHOR]

The ionic hydrogenation of N2 with H2 to give NH3 is investigated by means of density functional theory (DFT) computations using a cooperatively acting catalyst system. In this system, N2 binds to a neutral tungsten pincer complex of the type [(PNP)W(N2)3] (PNP=pincer ligand) and is reduced to NH3. The protons and hydride centers necessary for the reduction are delivered by heterolytic cleavage of H2 between the N2-tungsten complex and the cationic rhodium complex [Cp*Rh{2-(2-pyridyl)phenyl}(CH3CN)]+. Successive transfer of protons and hydrides to the bound N2, as well as all N xH y units that occur during the reaction, enable the computation of closed catalytic cycles in the gas and in the solvent phase. By optimizing the pincer ligands of the tungsten complex, energy spans as low as 39.3 kcal mol−1 could be obtained, which is unprecedented in molecular catalysis for the N2/H2 reaction system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Δ G=−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. [ABSTRACT FROM AUTHOR]