Ligand-Assisted Carbonyl Bond Activation in Single Diastereomeric Complexes on Platinum

Abstract

It is a significant challenge to relate ligand-assisted bond activation on metal surfaces to specific adsorption and intermolecular binding structures. To address this objective, we studied carbonyl bond activation in single chirality transfer complexes formed by methyl 3,3,3-trifluoropyruvate (i.e., MTFP) and (R)-1-(1-naphthyl)­ethylamine (i.e., (R)-NEA) on a Pt(111) surface. The experiments combined reflectance absorbance infrared spectroscopy (RAIRS), scanning tunneling microscopy (STM), and density functional theory (DFT) methods. While STM measurements, in combination with DFT calculations, permit the study of single surface complexes, RAIRS is an ensemble technique that yields a composite spectrum resulting from an often heterogeneous distribution of molecular structures on the sampled surface. We show that the intrinsic thermal behavior of the MTFP/(R)-NEA/Pt­(111) system facilitates meaningful comparison between single complex measurements by STM and ensemble measurements by RAIRS in that the vibrational signal can be attributed to a small number of complexation configurations, one of which has a high relative abundance. We take advantage of mode mixing in a ν­(CF3) + ν­(CO)keto vibration to detect a spectroscopic signature for complexation-induced carbonyl bond activation. A red-shift of the band correlates with DFT-predicted lengthening of the bridge-bonded carbonyl group. While the intensity of the shifted band is in the majority due to the most abundant complexation configuration, minority states produce line broadening. In addition to providing insight on rate-enhancement in enantioselective reactions on catalysts bearing chiral auxiliaries, the study contributes to the development of ligand control of reactivity and selectivity in heterogeneous catalysis.

Intermediates and Bimolecular Structures Formed through Thermal and Tip-Induced Partial Dehydrogenation of 2,2,2-Trifluoro-1-phenylethanol on Pt(111)

“…Although the red-shift cannot be related to a specific complexation structure, since multiple TFAP/1-(1-naphthyl)­ethylamine structures coexist, the single red-shifted band suggests that TFAP is in majority present in NEA/TFAP complexes rather than in hy -TFAP/TFAP structures. This analysis is based on a clearer example in a separate publication, where activation of the keto -carbonyl bond of methyl 3,3,3-trifluoropyruvate through complexation to NEA on Pt(111) was definitively demonstrated …”

“…This analysis is based on a clearer example in a separate publication, where activation of the ketocarbonyl bond of methyl 3,3,3-trifluoropyruvate through complexation to NEA on Pt(111) was definitively demonstrated. 34…”

Bioinspired Catalyst Design Principles: Progress in Emulating Properties of Enzymes in Synthetic Catalysts

“…The mechanisms of catalyst modification are based on steric effects, with hydrogen bonds between the substrates and the organic modifiers directing the substrate moiety that interacts with the metal. 192,193 There is little evidence, however, that the decoration of metal particles by organic molecules affects the nature of reactive hydrogen. 194 Functionalization of Pt with proline rendered the metal highly chemoselective in the hydrogenation of acetophenone without sacrificing activity.…”

“…The widely explored approach of combining metal nanoparticles with organic modifiers is one possibility for creating a proton transport network. − This approach also imparts chemoselectivity and enantioselectivity to metal surfaces. The mechanisms of catalyst modification are based on steric effects, with hydrogen bonds between the substrates and the organic modifiers directing the substrate moiety that interacts with the metal. , There is little evidence, however, that the decoration of metal particles by organic molecules affects the nature of reactive hydrogen . Functionalization of Pt with proline rendered the metal highly chemoselective in the hydrogenation of acetophenone without sacrificing activity .…”