Surface‐Assisted Cyclodehydrogenation; Break the Symmetry, Enhance the Selectivity

Abstract

Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.

On‐Surface Metalation and 2D Self‐Assembly of Pyrphyrin Molecules Into Metal‐Coordinated Networks on Cu(111)

“…The molecules appear then with slightly irregular shapes and regular self‐assembly is absent. A similar appearance was also observed for tetrapyrrole monolayers on Cu and Ag substrates which also dehydrogenated at the periphery by annealing to 500 – 640 K …”

“…Such pyrolysis is a common feature for adsorbed hydrocarbons that are too tightly bound to the surface to become desorbed intact. This process has also been observed for monolayers of tetrapyrroles adsorbed on metal surfaces …”

Survival of spin state in magnetic porphyrins contacted by graphene nanoribbons

“…However, as we shall show here, the CDH reaction can also affect the porphyrin core by forming new five-membered rings (blue and red arrows in Fig. 1C ), which fuse the pyrrole moieties to the graphenoid backbone and impose planarity to the tetrapyrrole core ( 21 , 22 ).…”

Stereoselective On‐Surface Cyclodehydrofluorization of a Tetraphenylporphyrin and Homochiral Self‐Assembly

“…A similar selectivity in cyclodehydrogenation has been only observed for 2H-tetraphenylporphyrin (TPP) due to its twofold symmetry but not for the fourfold-symmetric metalated TPP. [12] At lower coverages, however, the stereoselectivity towards product 5 is less pronounced. Figures 2 a-c show STM images of the final products, starting at coverages of 0.1 ML, 0.3 ML and 0.9 ML of 1, respectively.…”