An Account of Chiral Metal Surfaces and Their Enantiospecific Chemistry

Abstract

ConspectusMolecular chirality has been of scientific interest since 1848 when Pasteur demonstrated its direct connection to the rotation of light by solutions of chiral compounds. In the 1960s the connection was made between the chirality of pharmaceutical compounds and their physiological impact; one enantiomer can be therapeutic while the other is toxic. That realization prompted enormous effort in the synthesis of enantiomerically pure compounds for bioactive use (a $300B/yr market). Until relatively recently, metals were ignored as potential substrates for asymmetric surface chemistry because metals have highly symmetric, achiral bulk structures and the premise was that they could not expose chiral surfaces. In 1996, we demonstrated that the high Miller index surfaces of metals can be chiral, existing in two enantiomeric forms M(hkl)R&S, and we hypothesized that they exhibit enantiospecific interactions with chiral adsorbates. Most such intrinsically chiral metal surfaces have ideal structural motifs based on low Miller index terraces separated by kinked monatomic steps. This Account begins with a short tutorial on the ideal and real structures of chiral metal surfaces to provide a firm basis for understanding the origin of their chirality. It then chronicles the evolution of our understanding of their enantiospecific interactions with chiral adsorbates.Detecting, quantifying, and understanding enantiospecific surface chemistry on intrinsically chiral metal surfaces has been far more challenging than coming to the realization that such surfaces exist. The first successes came from measurements and modeling of the enantiospecific adsorption energetics of small chiral molecules such as propylene oxide and trans-1,2-dimethylcyclopropane. These revealed one of the core challenges to observing enantiospecificity, the fact that the enantiospecificities of reaction energetics and barriers tend to be small, i.e., a few kJ/mol. Measurements of the enantiospecific adsorption energetics of R-3-methylcyclohexanone on seven different Cu(hkl)R&S surfaces demonstrated their sensitivity to surface structure, but again revealed variations of only a few kJ/mol. One of the most important advances in our understanding of chiral surface chemistry is that the limitations imposed by weakly enantiospecific interactions can be circumvented by processes with nonlinear kinetics or equilibria. As an example, the surface explosion mechanism of d- and l-tartaric acid decomposition on Cu(hkl)R&S surfaces leads to enantiospecific rates that differ by almost 2 orders of magnitude, in spite of the fact that the rate constants are only weakly enantiospecific. More surprising is the observation that equilibrium adsorption of nonracemic mixtures of d- and l-aspartic acid can lead to autoamplification of enantiomeric excess, even on achiral Cu(111) surfaces. Again, this arises from a nonlinear adsorption isotherm. Most recently, we have developed a high throughput method for identification of the most enantiospecific surface orientation for a given reaction from the continuum of Cu(hkl)R&S surface orientations. These developments, and others described in this Account, firmly establish some of the basic principles of chiral surface chemistry.