Abstract
Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. In this study, we address this issue by using structurally comparatively simple, well-defined, and chiral intermetallic PdGa{111} surfaces as catalytic substrates. We demonstrate the impact of chirality transfer and ensemble effect for the thermally activated azide-alkyne Huisgen cycloaddition between 3-(4-azidophenyl)propionic acid and 9-ethynylphenanthrene on these threefold symmetric intermetallic surfaces under ultrahigh vacuum conditions. Specifically, we encounter a dominating ensemble effect for this reaction as on the Pd3-terminated PdGa{111} surfaces no stable heterocoupled structures are created, while on the Pd1-terminated PdGa{111} surfaces, the cycloaddition proceeds regioselectively. Moreover, we observe chirality transfer from the substrate to the reaction products, as they are formed enantioselectively on the Pd1-terminated PdGa{111} surfaces. Our results evidence a determinant ensemble effect and the immense potential of PdGa as asymmetric heterogeneous catalyst.
Highlights
Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts
Using scanning tunneling microscopy (STM), complemented with X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations, we demonstrate a significant ensemble effect on the PdGa{111} surface reactivity, manifested in the occurrence of the regio- and, most importantly, enantioselective cycloaddition between APA and 9-EP on the Pd1-terminated PdGa{111} surfaces, but its suppression on Pd3-terminated PdGa{111}
Depressions, which are absent on the pristine surface, are observed on the substrate, as indicated by the white arrow. We attribute these depressions to fragments of the decomposed carboxylic acid group of the APA molecule as they are observed upon deposition of ex situ synthesized 1,4triazoles (Fig. S1) and XPS investigations show a strong C 1s component arising from the carboxylic acid group (Fig. S2a, Table S1 and Supplementary Note 1)
Summary
Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. The catalog of on-surface reactions and synthesized structures broadened extensively, the focus rarely lay on enantioselective synthesis, which is of immense importance for instance in pharmaceutical, agricultural, or food industry[2,3] This lack of enantioselective on-surface synthesis originates largely from the scarcity of intrinsically chiral, catalytically active, and wellcharacterized single-crystal surfaces needed for this task. Chiral metal surfaces promise an increased thermal stability, a reduction of complexity arising from the multitude of molecule–molecule arrangements and interactions, and would not require enantiopure molecular modifiers, but are accompanied by a reduced adjustability towards specific reactions In this context, high Miller-index surfaces of achiral crystals[12,13], which exhibit a low density of chiral centers only at the kink sites of atomic steps separating adjacent terraces of low-index surfaces, have been established for enantioselective decomposition reactions under UHV conditions[14,15]. Owing to their differing surface terminations, in combination with their equal lattice parameters, identical symmetry, and similar electronic properties, the PdGa{111} surfaces are ideally suited to disentangle ensemble and ligand effects, i.e., the influence of the local geometric and electronic properties, respectively, in asymmetric heterogeneous catalysis[27,28,29,30]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
