Extended surface chirality from supramolecular assemblies of adsorbed chiral molecules

Abstract

The increasing demand of the chemical and pharmaceutical industries for enantiomerically pure compounds has spurred the development of a range of so-called 'chiral technologies' (ref. 1), which aim to exert the ultimate control over a chemical reaction by directing its enantioselectivity. Heterogeneous enantioselective catalysis is particularly attractive because it allows the production and ready separation of large quantities of chiral product while using only small quantities of catalyst. Heterogeneous enantioselectivity is usually induced by adsorbing chiral molecules onto catalytically active surfaces. A mimic of one such catalyst is formed by adsorbing (R,R)-tartaric acid molecules on Cu(110) surfaces: this generates a variety of surface phases, of which only one is potentially catalytically active, and leaves the question of how adsorbed chiral molecules give rise to enantioselectivity. Here we show that the active phase consists of extended supramolecular assemblies of adsorbed (R,R)-tartaric acid, which destroy existing symmetry elements of the underlying metal and directly bestow chirality to the modified surface. The adsorbed assemblies create chiral 'channels' exposing bare metal atoms, and it is these chiral spaces that we believe to be responsible for imparting enantioselectivity, by forcing the orientation of reactant molecules docking onto catalytically active metal sites. Our findings demonstrate that it is possible to sustain a single chiral domain across an extended surface--provided that reflection domains of opposite handedness are removed by a rigid and chiral local adsorption geometry, and that inequivalent rotation domains are removed by successful matching of the rotational symmetry of the adsorbed molecule with that of the underlying metal surface.

Homochiral Conglomerates and Racemic Crystals in Two Dimensions: Tartaric Acid on Cu(110)

“…However, the (9 0, AE 1 2) lattice must be explained as a superposition of the two enantiomorphous bitartrate lattices of the pure enantiomers. [18] Therefore, we conclude that this LEED pattern reflects a lattice structure in which the enantiomers are laterally separated into homochiral (9 0, 1 2)-(R,R)-TA and (9 0, À1 2)-(S,S)-TA domains on the surface. The primary electron beam probes an area that contains both enantiomorphous homochiral lattices.…”

The Chemistry of Intrinsically Chiral Surfaces

“…In recent years single crystal model systems representing both chirally modified and intrinsically chiral surfaces have been studied with surface science methods in some detail, mainly by scanning tunnelling microscopy (STM), photoemission spectroscopies (XPS, PhD, NEXAFS), IR spectroscopy (RAIRS) and density functional theory (DFT) (see [4,[11][12][13] and references therein). Due to the choice of experimental methods and the complexity of these systems there is relatively little known to date about the exact adsorption geometries on such surfaces although structural information is crucial for the understanding of stereoselectivity in any context.…”

First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands

“…Chiral domains are then formed on the surface with equal abundance of left- and right-handedness. This local chirality can be observed by scanning tunnelling microscopy4.…”

“…For example, achiral molecules may turn chiral on adsorption on a surface, even if the latter itself is not chiral123. In addition, achiral molecules can form chiral patterns on achiral surfaces4. This emergence of chirality is due to the restriction of the molecules to two-dimensional space on adsorption.…”