Carbon-based cyclic molecules are important, e.g., as ligands in organometallic chemistry. Cyclooctatetraene (COT), for example, features an eight-membered ring with four C=C bonds and can form organometallic complexes and sandwich compounds. Cyclic oligo-phosphorus compounds are related compounds that are generally more challenging to synthesize. Oligo-phosphorus species can be stabilized, e.g., by organic substitution or by coordination to transition metals. So far, no monocyclic all-phosphorus species with more than six ring atoms had been found.

Manfred Scheer, University of Regensburg, Germany, and colleagues have synthesized the first complex containing a cyclooctatetraene-like cyclo-P₈ ligand (pictured). The team started from Cp”’Ta(CO)₄ (Cp”’ = 1,2,4-^tBu₃C₅H₂), which underwent co-thermolysis with white phosphorus (P₄) in boiling 1,3-diisopropylbenzene (DIB). This reaction gave the cyclo-P₈-containing complex [{Cp”’Ta}₂(μ,η^2:2:2:2:1:1-P₈)] in a yield of 41 %.

The team studied the reactivity of the complex and found that the P₈ unit can coordinate further metal-containing fragments such as W(CO)₅ units. Oxidation of the complex leads to the formation of a P–P bond and dimerization to a dicationic species. A reduction of the complex cleaves a P–P bond, followed by dimerization to give a dianionic species. According to the researchers, the oxidation and reduction products contain the largest cationic polyphosphorus cage species and the longest unsubstituted polyphosphorus chain so far, respectively.

• Synthesis and Reactivity of a Cyclo‐Octatetraene‐Like Polyphosphorus Ligand Complex [Cyclo‐P₈],
  Christoph Riesinger, Fabian Dielmann, Robert Szlosek, Alexander Virovets, Manfred Scheer,
  Angew. Chem. Int. Ed. 2023.
  https://doi.org/10.1002/anie.202218828