Molecular recognition in adsorption and electro-oxidation at chiral platinum surfaces

Abstract Representatives of five classes of optically active compounds have been adsorbed at chiral single crystal Pt electrodes in order to assess the structural and other requirements for chiral/chiral molecular recognition in adsorption and in possible subsequent electro-oxidation. In some cases, analogous studies have been made using achiral stepped and flat Pt surfaces. Cyclic voltammetry in 0.1 M sulphuric acid, which reveals the presence of terraces, steps and kinks at the Pt surface, has been used to follow adsorption as a function of surface geometry and to detect electro-oxidation. For the alkaloids cinchonidine and codeine, and various aromatic compounds possessing chirality in the side-chain (including ephedrine) indiscriminate adsorption occurred at comparable rates at kinked, stepped, and flat surfaces; a proportion of the Pt remained exposed at maximum coverage due to the nature of the random ad-layer. Adsorption of anions of R , R -tartaric acid caused a shift to negative potential of the hydrogen underpotential features and the {1 1 1} anion feature of the cyclic voltammograms, showing that tartrate ions were more strongly adsorbed than sulphate and bisulphate ions. Adsorption of R - and S -2-chloropropanol, R - and S -2-butanol, R -1,3-butanediol, and 2 R ,3 R - and 2 S ,3 S -butanediol was accompanied in each case by significant oxidation currents in the double layer potential range, indicating that electro-oxidative adsorption had occurred. However, both the magnitudes of the currents and the potentials at which maximum current was observed were independent of the chirality ( R - or S -) of the Pt surfaces. This contrasts with the enantioselective response observed when d - and l -glucose were adsorbed at R - and S -Pt(3 2 1). The requirements for chiral recognition in adsorption appear to be (i) the dissociative adsorption of a weakly adsorbed precursor with the chiral surface and (ii) a geometry which is favoured by the adsorbate having a cyclic structure or containing an aldehyde moiety adjacent to a stereogenic centre. The importance of molecular sticking probability and/or adsorption enthalpy is also discussed. It is concluded that as the sticking probability approaches unity or the adsorption enthalpy becomes large relative to the difference in adsorption energy between R - and S -kinks, enantioselective effects will become more and more obscured.