Phosphine-free ruthenium NCN-ligand complexes and their use in catalytic CO2 hydrogenation

This work investigates the hydrogenation of carbon dioxide to formate catalysed by the phosphine-free Ru complexes Ru(OtBu)(κ³-NCN)(^tBubpy) and RuH(OtBu)(κ²-NCN)(^tBubpy) (OtBu = tert-butoxide, κ²-NCN = 1,3-di(2-methylpyridyl)-4,5-diphenyl-1H-imidazol-2-ylidene, where one pyridyl moiety is not coordinated to Ru, ^tBubpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). A catalytic cycle is proposed for this reaction, supported by computational studies and the characterization of the hydride and the formate intermediates proposed to be involved. Modest catalytic turnovers are demonstrated at relatively low pressures and temperatures. The proposed rate determining step is heterolytic H₂ splitting to regenerate the Ru-H complex, which has an estimated hydricity of approx. 27 kcal mol^-1. The κ²-NCN ligand in the hydride complex undergoes a variety of dynamic processes as detected by EXSY spectroscopy including a pyridyl "roll-over" carbon-hydrogen - ruthenium hydride exchange, possibly occuring via a Perutz-Sabo-Etienne CAM mechanism.

Structural inhomogeneity as a factor promoting the homogenous catalysis of CO2 hydrogenation by (PMe3)4RuH2

During homogenous catalysis by organometallic complexes, the dissociation of a ligand to produce an unsaturated site on the metal center is often invoked as the first step of activation, especially when photo-excitation is involved. In this theoretical study, we demonstrated that under mild conditions, a thermodynamically unstable yet dynamically favorable active intermediate could be produced by the inhomogeneity of the solvent distribution around the catalyst rather than by ligand dissociation. This occurred at the end of the first catalytic cycle when the product was eliminated. The empty site was immediately filled by one of the additive molecules aggregated around the reaction center even when the intermediate complex was unstable, producing a transient and more active catalyst. This process accounted for the accelerated reaction rate observed in the landmark CO₂ hydrogenation catalyzed by (PMe₃)₄RuH₂ in supercritical CO₂ when H₂O, MeOH, or HNMe₂ was added. This also suggests a new way to exploit the structural inhomogeneity around an organometallic complex for the design of superior catalysts.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

Surface organometallic chemistry in heterogeneous catalysis

Surface organometallic chemistry has been reviewed with a special focus on environmentally relevant transformations (C–H activation, CO₂conversion, oxidation).