Berlin 2018 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 74: Nanostructures at surfaces: Other aspects
O 74.1: Vortrag
Mittwoch, 14. März 2018, 16:45–17:00, MA 141
Asymmetric coupling on the chiral PdGa{111} surfaces — Samuel Stolz1,2, Oliver Gröning1, Harald Brune2, and •Roland Widmer1 — 1Empa, nanotech@surfaces, 8600 Dübendorf, Switzerland — 2EPFL, ICMP, 1015 Lausanne, Switzerland
Intermetallic PdGa exists in two enantiomeric crystal forms A and B due to its P213 space group and all its surfaces are chiral. Furthermore, the (111) and (-1-1-1) surface terminations of the same crystal form are structurally different. In particular, one is terminated by a single, isolated Pd atom (Pd1), while the other reveals isolated Pd trimers (Pd3). We demonstrated a 98% enantioselectivity of Pd1 and Pd3 by the adsorption of prochiral 9-Ethynylphenanthrene (9-EP) [1].
To profit from this high enantioselectivity we aimed to perform highly chiral asymmetric, covalent coupling reactions and have chosen an Azide-Alkine Huisgen Cycloaddition. Therefore, 9-EP as the alkine and 3-(4-Azidophenyl)propionic acid as the azide, were co-adsorbed on Pd1 and Pd3. The catalysed reaction is regiostereoselective on Pd1 and one out of four reaction products is favoured over the others yielding in an enantiomeric excess of up to 66%.
On the other hand, no reaction between 9-EP and the azide was observed on Pd3. Therefore, the reactivity for this particular reaction not only the depends on the d-band center, but much stronger on the surface geometry [2] as evidenced by the ensemble effect [1].
[1] J. Prinz, EPFL Thèse N°6337 (2014).
[2] J. K. Nørskov et al., Chem. Soc. Rev. 37 (2008) 2163-2171.