Influence of modification pH and temperature on the interaction of methylacetoacetate with (S)-glutamic acid-modified Ni{111}

The adsorption of (S)-glutamic acid onto Ni{111} from solution was investigated with reflection absorption infrared spectroscopy as functions of modification temperature and pH. Modification at 300 K results in the formation of approximately one monolayer of adsorbate. The extent of protonation of the adsorbate decreases with increasing pH. Modification at 350 K produces a thicker adsorbate layer, which is assigned to nickel glutamate (low pH) and a mixture of nickel and sodium glutamate (high pH). Adsorption of methylacetoacetate onto the chirally modified surfaces occurs predominantly in the diketone tautomeric form when adsorbed glutamic acid exists in the protonated form. By contrast, methylacetoacetate exists primarily in the enol tautomeric form when glutamate (i.e., anionic) species are present. The implications of our findings for understanding the mechanism of Ni-catalyzed beta-ketoester hydrogenation are discussed.