Abstract
Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for the production of enantiopure pharmaceuticals. One of the major current challenges in heterogeneous chiral catalysis is the fundamental-level understanding of how such chirally-modified surfaces interact with chiral and prochiral molecules to induce their enantioselective transformations. Herein we report the first direct calorimetric measurement of the adsorption energy of chiral molecules onto well-defined chirally-modified surfaces. Two model modifiers 1-(1-naphthyl)ethylamine and 2-methylbutanoic acid were used to impart chirality to Pt(111) and their interaction with propylene oxide was investigated by means of single-crystal adsorption calorimetry. Differential adsorption energies and absolute surface uptakes were obtained for the R- and S-enantiomers of propylene oxide under clean ultrahigh vacuum conditions. Two types of adsorption behavior were observed for different chiral modifiers, pointing to different mechanisms of imparting chirality to metal surfaces. The results are analyzed and discussed in view of previously reported stereoselectivity of adsorption processes.
Highlights
IntroductionChiral differentiation can be generally accomplished on three types of metal surfaces: (i) intrinsically chiral terminations of higher Miller index surfaces; (ii) surfaces templated by ordered structures of chiral modifiers (this type of chirality is frequently referred to as ‘‘supramolecular chirality’’) and (iii) the surfaces carrying individual chiral modifiers which form 1 : 1 complexes with pro-chiral reactant molecules
Chiral differentiation can be generally accomplished on three types of metal surfaces: (i) intrinsically chiral terminations of higher Miller index surfaces; (ii) surfaces templated by ordered structures of chiral modifiers and (iii) the surfaces carrying individual chiral modifiers which form 1 : 1 complexes with pro-chiral reactant molecules
These chiral modifiers were chosen as potential proxies for two different mechanisms of imparting chirality to achiral metal surfaces: a templating mechanism (MBA) based on the formation of chiral overlayers and a docking mechanism (NEA) based on the formation of 1 : 1 complexes between the chiral modifier and the adsorbate molecules
Summary
Chiral differentiation can be generally accomplished on three types of metal surfaces: (i) intrinsically chiral terminations of higher Miller index surfaces; (ii) surfaces templated by ordered structures of chiral modifiers (this type of chirality is frequently referred to as ‘‘supramolecular chirality’’) and (iii) the surfaces carrying individual chiral modifiers which form 1 : 1 complexes with pro-chiral reactant molecules. In the case of asymmetric surfaces, on which chirality is imposed by supramolecular assemblies of chiral adsorbates, the stereoselective adsorption sites are produced by so called ‘‘chiral pockets’’ in the ordered chiral overlayers. The interaction of chiral molecules with the chiral pockets in such supramolecular chiral surfaces has been extensively investigated by Tysoe’s group.. R- and S-propylene oxides (PO) were adsorbed on 22726 | Phys.
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