Metal–ligand
cooperative properties of a bis-N-heterocyclic
carbene ruthenium CNC pincer catalyst and its activity in CO2 hydrogenation to formates were studied by DFT calculations complemented
by NMR spectroscopy and kinetic measurements. The dearomatized Ru–CNC*
pincer (1*) is significantly more reactive toward metal–ligand
cooperative activation of H2 and CO2 than the
structurally related phosphine-based Ru–PNP complex. The enhanced
reactivity of Ru–CNC* stems from the combination of electronic
properties of this system and the reduced geometric constraints imposed
onto the Ru center by the large and flexible CNC chelate. Heterolytic
dissociation of H2 by 1* results in the bis-hydrido
complex 2 that is active in hydrogenation of CO2. However, under commonly applied reaction conditions, the catalyst
rapidly deactivates via metal–ligand cooperative paths. The
transient formation of the dearomatized complex Ru–CNC* (1*) in the course of the reaction leads to the irreversible
cooperative activation of CO2, resulting in the stable
adduct 3 that is not catalytically competent. By an increase
in the H2/CO2 ratio, this deactivation path
can be effectively suppressed, resulting in a stable and rather high
catalytic performance of Ru–CNC.
Recent synthetic efforts aimed at reconstructing the beginning of life on our planet point at the plausibility of scenarios fueled by extraterrestrial energy sources. In the current work we show that beyond nucleobases the sugar components of the first informational polymers can be synthesized in this way. We demonstrate that a laser-induced high-energy chemistry combined with TiO2 catalysis readily produces a mixture of pentoses, among them ribose, arabinose and xylose. This chemistry might be highly relevant to the Late Heavy Bombardment period of Earth’s history about 4–3.85 billion years ago. In addition, we present an in-depth theoretical analysis of the most challenging step of the reaction pathway, i.e., the TiO2-catalyzed dimerization of formaldehyde leading to glycolaldehyde.
Reaction mechanisms for the catalytic hydrogenation of CO2 by faujasite‐supported Ir4 clusters were studied by periodic DFT calculations. The reaction can proceed through two alternative paths. The thermodynamically favoured path results in the reduction of CO2 to CO, whereas the other, kinetically preferred channel involves CO2 hydrogenation to formic acid under water‐free conditions. Both paths are promoted by catalytic amounts of water confined inside the zeolite micropores with a stronger promotion effect for the reduction path. Co‐adsorbed water facilitates the cooperation between the zeolite Brønsted acid sites and Ir4 cluster by opening low‐energy reaction channels for CO2 conversion.
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