In Situ Structural Determination of a Homogeneous Ruthenium Racemization Catalyst and Its Activated Intermediates Using X‐Ray Absorption Spectroscopy

Abstract

The activation process of a known Ru‐catalyst, dicarbonyl(pentaphenylcyclopentadienyl)ruthenium chloride, has been studied in detail using time resolved in situ X‐ray absorption spectroscopy. The data provide bond lengths of the species involved in the process as well as information about bond formation and bond breaking. On addition of potassium tert‐butoxide, the catalyst is activated and an alkoxide complex is formed. The catalyst activation proceeds via a key acyl intermediate, which gives rise to a complete structural change in the coordination environment around the Ru atom. The rate of activation for the different catalysts was found to be highly dependent on the electronic properties of the cyclopentadienyl ligand. During catalytic racemization of 1‐phenylethanol a fast‐dynamic equilibrium was observed.

Operandomonitoring of mechanisms and deactivation of molecular catalysts

“…Recently, Bäckvall's group published a work of operando XAS to study the activation of a ruthenium racemization catalyst. 68 X-ray absorption spectroscopy enabled the inspection of the stereostructure coordination environment around ruthenium during activation, as well as the proposed acyl intermediate and the activated alkoxide complexes for different molecular ruthenium catalysts, providing a valuable foundation for efforts to further catalyst development. Since XAS analysis requires structural models which hinder it to discriminate elements from the same periodic row, a combination with other structural and analytical techniques, such as NMR, can make it more effective.…”

The Real Structure of Pd(OAc)2 in Various Solvents†

“…Due to the long term projects on using the technique of XAFS to explore the reaction mechanism of catalytic reactions under homogenous conditions, we are very interested in the “real” structures of metal complexes in solution. [ 9 ] XAFS is a powerful technique to study the structure of metal complexes in homogeneous solution, [ 9‐10 ] which can supply not only the valence of the metal center but also the information of the coordination environment. Compared with the commonly used X‐ray crystal structure analysis, XAFS can provide structural information under real reaction conditions, which should be more instructive especially in the cases where no crystal could be obtained.…”

Progress on the Stereoselective Synthesis of Chiral Molecules Based on Metal-Catalyzed Dynamic Kinetic Resolution of Alcohols with Lipases

“…The reaction proceeds through beta-hydride elimination of metal-coordinated alkoxide 2c. The resulting Ru-hydride Complex 2e forms after the dissociation of a CO ligand as suggested by 13 CO exchange studies and DFT investigations [66]. Racemization process continues with hydride re-addition to the Ru-coordinated ketone followed by CO coordination.…”

“…The catalytic cycle of Complex 2 begins with its instantaneous activation promoted by tBuOK with the assistance of the Ru-coordinated CO (Scheme 22a) [13]. Experimental evidences based on X-ray absorption spectroscopy confirmed the formation of the acyl-intermediate 2a [66]. The subsequent alkoxide migration from the carbon atom to the metal produces the catalytically active Complex 2b which undergoes alkoxide/(S)−1phenylethanol exchange.…”