Highly Structure‐Selective On‐Surface Synthesis of Isokekulene Versus Kekulene

On-surface synthesis has allowed for the preparation of many novel molecular structures with intriguing properties. Here, we present a case of highly selective on-surface synthesis of the previously unknown cycloarene isokekulene on Cu(110) and the prototypical cycloarene kekulene on Cu(111) from the very same specialized precursor prepared in solution.

Abstract

The role of different facets of metal nanoparticles in steering reaction pathways is crucial for the design of heterogeneous catalysts with superior selectivity. As a prominent class of reactions, transition-metal-catalyzed carbon-hydrogen (C─H) bond activation is widely used for the synthesis of base chemicals, modern organic materials, and pharmaceuticals. Here, we report orthogonal selectivity in intramolecular cyclodehydrogenation of a nonplanar cyclic precursor steered by different facets of a copper single crystal. On the Cu(110) surface, the previously unknown cycloarene isokekulene forms with a high selectivity of 92%, whereas reaction on the Cu(111) surface exclusively yields kekulene (>99%). Combining scanning tunneling microscopy with CO-functionalized tips and density functional theory, we identify two adsorption geometries of the precursor, which react to the respective products. Isokekulene adopts two nonplanar adsorption configurations and exhibits strong molecule-substrate interactions, explaining its preferential formation on Cu(110). This combined in-solution and on-surface synthesis approach represents an alternative route for the highly selective synthesis of molecules that are challenging to synthesize and process via conventional methods.

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