Kinetic and Scanning Transmission Electron Microscopy Investigations on a MCM-41 Supported Cluster Derived Enantioselective Ruthenium Nanocatalyst

Abstract

The asymmetric hydrogenation of methyl pyruvate to methyl lactate, by cinchonidium functionalized MCM-41 supported [Ru4(μ-H)3(CO)12]− as the precatalyst has been studied kinetically and by scanning transmission electron microscopy (STEM). Existence of an induction time and two competitive equilibriums are inferred from the time monitored conversion data. Steady state approximation gives a poor fit, but a kinetic model (Eley−Rideal) consisting of a fast equilibrium between methyl pyruvate and the catalyst, a slow one between the catalyst and dihydrogen, and a rate determining reaction between the latter and methyl pyruvate, accurately simulates the time monitored conversion profiles. The model suggests that on increasing the methyl pyruvate concentration there is a change in the stoichiometry of the equilibrium between the catalyst and the methyl pyruvate. The change in enantioselectivity with time can also be accurately simulated by assuming enantiomeric excess to be proportional to the rate constant for methyl lactate formation. Both kinetic and STEM data strongly suggest that in the fresh catalyst the bare metal cluster framework is retained, and under the catalytic conditions agglomeration of the subnano sized clusters leading to the formation of nanoparticles of ruthenium is a relatively slow process. A hypothetical enantionface selection mechanism consistent with the empirical rate law, previous reports, and other experimental evidence is proposed.

Chemoselective Hydrogenation and Transfer Hydrogenation of Olefins and Carbonyls with the Cluster‐Derived Ruthenium Nanocatalyst in Water

“…In nanocatalysis, the dependence of selectivity of a given reaction on specific crystal planes, particle size, and nature of the stabilizer is well established 8. 35–37 We are unable to record high‐angle annular dark field scanning TEM data of 3 ; however, as mentioned earlier, HRTEM images show nearly monodisperse crystalline particles with predominantly Ru(1 1 1) crystal planes (Figure 1).…”

“…Various techniques such as extended X‐ray absorption fine structure have shown that in inorganic oxide‐supported carbonyl cluster‐derived nanocatalysts, the essential integrity of the original polyhedral structure is retained in the decarbonylated form 33. By using high‐angle annular dark‐field scanning TEM and kinetic analyses, we showed that on functionalized MCM‐41‐supported [H 3 Ru 4 (CO) 12 ] − ‐derived nanoparticles, the tetrahedral Ru 4 framework is retained 8. There is enough evidence to show that in solution, subnanometer Rh 4 clusters with partial coordinative unsaturation are relatively robust catalysts.…”

“…In our earlier work, we have shown that ion pairing of anionic carbonyl clusters with tetraalkyl ammonium groups of functionalized inorganic oxides or organic polymers is a general method for the synthesis of nanocatalyst precursors 8–14. By using water‐soluble poly(diallyldimethylammonium chloride) (PDADMAC, 1 ) as the stabilizer, we have also reported platinum‐ and ruthenium‐based nanocatalysts 15–18.…”

“…Nanoparticle catalysis in green solvents such as water has shown tremendous potential, and systematic guidelines for the further development of metal cores, stabilizers, and solvents have been presented 7. Given the above‐mentioned background and our own involvement with nanocatalysis over the years, we wanted to evaluate the potential of the water‐soluble ruthenium precatalyst 2 (see Scheme ) in a broad range of hydrogenation and transfer hydrogenation reactions 8–18. We also wanted to determine whether the literature reported “on water” effect, that is, improved rate in water, could be observed in these reactions.…”

Carbon Monoxide Assisted Self‐Assembled Platinum Nanoparticles for Catalytic Asymmetric Hydrogenation

“…In this reaction, the catalyst systems are mainly developed on a solid support. Previously, we have reported the enantioselective hydrogenation of ketoester and ketone using [Pt 15 (CO) 30 ] 2− and [H 3 Ru 4 (CO) 12 ] − cluster‐derived supported catalysts modified with functionalized cinchonidine 21–24. In this context, asymmetric hydrogenation by non‐supported Pt NPs, especially with Pt NRs as catalyst, is of special interest due to the high activity, selectivity, and large surface area of nanostructured materials.…”