Abstract
Understanding the interaction of amino acids with metal surfaces is essential for the rational design of chiral modifiers able to confer enantioselectivity to metal catalysts. Here, we present an investigation of the adsorption of aspartic acid (Asp) on the Ni{100} surface, using a combination of synchrotron X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure, and density functional theory simulations. Based on the combined analysis of the experimental and simulated data, we can identify the dominant mode of adsorption as a pentadentate configuration with three O atoms at the bridge sites of the surfaces, and the remaining oxygen atom and the amino nitrogen are located on atop sites. From temperature-programmed XPS measurements, it was found that Asp starts decomposing above 400 K, which is significantly higher than typical decomposition temperatures of smaller organic molecules on Ni surfaces. Our results offer valuable insights into understanding the role of Asp as a chiral modifier of nickel catalyst surfaces in enantioselective hydrogenation reactions.
Highlights
Over the last decades, there has been a growing demand for optically pure drugs and compounds.[1]
One of the best-studied examples in this context is the chiral modification of Ni catalysts with tartaric acid (TA) or amino acids to enable enantioselective hydrogenation of β-ketoesters, such as methyl acetoacetate (MAA).[2−4,6,7] This reaction system is well characterized in terms of macroscopic parameters, for example, temperature, pH, reaction yields, and enantiomeric excess, but still poorly understood in terms of molecular level interactions
They are compatible with spectra reported by Karagoz et al for Asp on Cu{100}40 and spectra for other amino acids on transition metal surfaces.[15,16,32−39] Spectra were recorded after successively dosing Asp in 5 min intervals onto the Ni{100} surface held at 250 K
Summary
There has been a growing demand for optically pure drugs and compounds.[1]. The modifier molecules act as precursors that interact with and modify the catalyst metal surface, conferring a chiral behavior that enhances the production of one enantiomer over the other during the catalytic reaction. One of the best-studied examples in this context is the chiral modification of Ni catalysts with tartaric acid (TA) or amino acids to enable enantioselective hydrogenation of β-ketoesters, such as methyl acetoacetate (MAA).[2−4,6,7] This reaction system is well characterized in terms of macroscopic parameters, for example, temperature, pH, reaction yields, and enantiomeric excess, but still poorly understood in terms of molecular level interactions. It has been hypothesized that chiral modification incorporates physical effects, such as substrate reconstruction and/or supramolecular assemblies of the modifier with solvents.[4,7−9] The latter idea is supported by studies that show that both the reaction rate and optical yield are significantly affected by the polarity of the solvent and the pH used in the modification
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