Solvation scientists Elsa Sanchez-Garcia, Wolfram Sander, and Karina Morgenstern were able, for the first time, to directly image a highly reactive carbene on a metal surface using scanning tunneling microscopy (STM). The research offers a glimpse on how carbenes interact with metal surfaces at the atomic level, a knowledge that could lead to the design of tailored catalytic systems. The findings have been published in the renowned journal JACS, where they have been featured as JACS Spotlight and with a front cover.
Carbenes are very reactive and versatile intermediates in organic synthesis and catalysis – their reactivity stemming from two electrons that are not shared with other atoms. In heterogeneous catalysis, the high reactivity of carbenes can be “tamed” by attaching them to metal surfaces, usually coinage metals like silver. Yet, the atomic details of carbenes’ interaction with the metal surface and its effects on reactivity were unknown.
Elsa Sanchez-Garcia from Duisburg-Essen University, together with Wolfram Sander and Karina Morgenstern from Ruhr University Bochum, used a metal surface of Ag(111) and deposited on it 9-diazofluorene (DAF), a carbene precursor. DAF, upon irradiation with UV-light, leads to fluorenylidene (FY), a simple aromatic carbene with a rigid planar structure. Scanning tunneling microscopy at low temperature (-265 °C) and ultrahigh vacuum (less than 2*10-10 mbar), then lead to the first characterization of this carbene-silver complex.
Images show that FY is tilted with respect to the surface and has a strong interaction with it that hinders usual FY reactions such as dimerization and formation of complexes if water is added. Further insights into the stabilization of such complexes, including the electronic origin of the FY-surface complex formation, were obtained by means of quantum mechanics calculations. Due to the complexity and size of the carbene-surface systems, the interactions were modelled by combining density functional theory, pseudopotentials, and plane waves. Partial electron transfer from the silver surface to the carbene center explains the strong binding, which reduces the electrophilicity and, in turn, the reactivity of FY. The authors suggest that, since the binding of carbenes on silver is usually less strong than with other coinage metals, it could be possible to modulate the carbene reactivity by changing the metal surface and carbene center substituents.
Original publication: Joel Mieres-Perez, Karsten Lucht, Iris Trosien, Wolfram Sander, Elsa Sanchez-Garcia, Karina Morgenstern:Controlling Reactivity—Real-Space Imaging of a Surface Metal Carbene, in: J. Am. Chem. Soc., 2021, DOI: 10.1021/jacs.0c12995