On-surface synthesis can be a useful approach to the generation and visualization of unstable or unusual molecules. For example, André Schirmeisen, Peter R. Schreiner, University of Gießen, Germany, and colleagues have performed an on-surface synthesis of the elusive cyclotriphosphazene (P₃N₃), an inorganic analogue of benzene. This compound had previously been prepared through thermal decomposition of P₃N₅ and by ionizing PH₃–NH3 ices with hard radiation, but was only partially characterized.

The team has developed a reliable method to generate P₃N₃ on Cu(111) and Au(111) surfaces via a voltage pulse-induced detachment of six chlorine substituents from the commercially available precursor hexachlorophosphazene (P₃N₃Cl₆) using a scanning tunneling microscopy (STM) tip (reaction pictured below). They used a simple procedure to trigger the dechlorination of the precursor on Cu(111). The STM tip was first positioned over the center of the target molecule, then the STM feedback was cut off, and a short voltage pulse was applied. Then, a new STM image was taken to visualize the resulting structural changes of the molecule. The compound was also generated on an Au(111) surface via similar STM manipulations.

 

 

The researchers found that the complete dechlorination of the precursor occurs in multiple steps. Several partially dechlorinated intermediates were observed. The product was characterized using high-resolution STM and atomic force microscopy (AFM) imaging, confirming the expected D_3h-symmetric six-membered ring structure. The team also photochemically synthesized P₃N₃ from an alternative precursor with six azide groups, P₃N₂₁, under matrix isolation conditions and characterized it using IR and UV/Vis spectroscopy. The analytical results, supported by density functional theory (DFT) calculations, indicate that the product is a 6π aromatic species with a singlet ground state.

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• On‐Surface Synthesis and Real‐Space Visualization of Aromatic P₃N₃,
  Qigang Zhong, Artur Mardyuvok, Ephrat Solel, Daniel Ebeling, Andre Schirmeisen, Peter Richard Schreiner,
  Angew. Chem. Int. Ed. 2023.
  https://doi.org/10.1002/anie.202310121