Influence of modification conditions on the interaction of methylacetoacetate with (R,R)-Tartaric acid-modified Ni{111}

Ni catalysts modified by the adsorption of the chiral molecule, (R,R)-tartaric acid, are known to be capable of catalyzing the enantioselective hydrogenation of beta-ketoesters (e.g., methylacetoacetate).(1) The adsorption of (R,R)-tartaric acid onto Ni{111} from solution was investigated with reflection absorption infrared spectroscopy as functions of modification temperature and pH. Under all conditions tested, RAIR spectra are consistent with the formation of several layers of nickel and/or sodium bitartrate. After washing, the majority of the overlayer is dissolved, leaving submonolayer quantities of adsorbate. At 350 K, the optimum modification temperature in terms of catalytic enantioselectivity, the washed surface exhibits a proportionately greater hydrogen tartrate content than at 300 K. When the modified surfaces are exposed to methylacetoacetate, the presence of the protonated tartrate species correlates with an enhancement in the diketone to enol tautomeric ratio of methylacetoacetate. Similarly, when the promoter NaBr is added to the modification solution, although little difference is observed in the RAIR spectra of adsorbed tartrate, there is a further increase in the diketone: enol ratio after exposure to methylacetoacetate. The implications for understanding the behavior of the enantioselective nickel catalyst are discussed.