Creating chiral surfaces for enantioselective heterogeneous catalysis: R,R-Tartaric acid on Cu(110)

One of the most successful ways of inducing enantioselectivity in a heterog eneous catalytic system is by the adsorption of chiral "modifier" molecules on the reactive metal surface. However, little is known about the nature o f the active sites present on the modified metal surface and how such modif iers bestow chirality to an achiral metal surface. In this paper we report the behavior of R,R-tartaric acid adsorption on a Cu(110) surface using hig h-resolution surface analytical techniques. R,R-Tartaric acid is known to b e an extremely successful modifier molecule for the enantioselective hydrog enation of methyl acetoacetate, the simplest beta-keto ester, to the R-enan tiomer of the product molecule methyl S-hydroxybutyrate. A combination of l ow-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Fourier transform reflection-absorption infrared spectroscopy (FT-RAIR S) techniques has allowed us to demonstrate that a complicated adsorption p hase diagram exists for this system. A rich variety of ordered overlayer st ructures are produced, in which preferred molecular forms, bonding and orie ntations of the chiral molecules are adopted, dependent on coverage, temper ature and time. These different adlayers will clearly play a different role in the enantioselective reaction. Of particular interest is the fact that under certain conditions, the 2-dimensional order of the IZ,R-tartaric acid adlayer destroys all symmetry elements at the surface, leading to the crea tion of extended chiral surfaces! Such chiral surfaces may be an important factor in defining the active site in heterogeneous enantioselective reacti ons.