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We report the synthesis, characterization and reactivity of an air-stable, well-defined acenaphthoimidazolylidene palladium–BIAN–NHC chloro dimer complex, [Pd(BIAN–IPr)(μ-Cl)Cl] 2 . This rapidly activating catalyst merges the reactive properties of palladium chloro dimers, [Pd(NHC)(μ-Cl)Cl] 2 , with the attractive structural features of the BIAN framework. [Pd(BIAN–IPr)(μ-Cl)Cl] 2 is the most reactive Pd( ii )–NHC precatalyst discovered to date undergoing fast activation under both an inert atmosphere and aerobic conditions. The catalyst features bulky-yet-flexible sterics that render the C–H substituents closer to the metal center in combination with rapid dissociation to monomers and strong σ-donor properties. [Pd(BIAN–IPr)(μ-Cl)Cl] 2 should be considered as a catalyst for reactions using well-defined Pd( ii )–NHCs.
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On the Origin of the Rate Enhancement by a R‐(+)‐1‐(1‐Naphthyl)‐Ethylamine‐Modified Pd(111) Model Catalyst for Methyl Pyruvate Hydrogenation
Abstract Chiral modifiers of heterogeneous catalysts can function as activity promotors to minimize the influence of unmodified sites on the enantiomeric excess to obtain highly enantioselective catalysts. However, the origin on this effect is not well understood. It is investigated using a model catalyst of R‐(+)‐1‐(1‐naphthyl)‐ethylamine (R‐1‐NEA)/Pd(111) for the hydrogenation of methyl pyruvate (MP) to methyl lactate (ML). The activity of the model catalyst remains constant for multiple turnovers. No rate enhancement is found for R‐1‐NEA coverages below ∼0.5 monolayer (ML), but a significant increase is found at R‐1‐NEA coverages of ∼0.75 ML, with a rate approximately twice that of the unmodified catalyst. This is investigated using infrared spectroscopy to distinguish between MP monomers and dimers. MP titration experiments with hydrogen show a half‐order hydrogen pressure dependence, with the monomer reacting at twice the rate as the dimer. It is found that the dimer is the most abundant species on clean Pd(111), but the ratio of monomers to dimers increases as the R‐1‐NEA coverage increases due to surface crowding. The monomeric species is also found to be more stable on the crowded surface than on clean Pd(111); the chiral modifier also serves to stabilize the reactant. Finally, this model nicely explains the unusual 1‐NEA‐covergae dependence of the reactivity.
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- Award ID(s):
- 1855199
- PAR ID:
- 10390441
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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