Biradicals and biradicaloids are highly reactive species that can occur in the processes of bond formation and bond breaking. They were discussed as intermediates even in Diels-Alder reactions. Hence, the study of biradicaloids is of general importance.

Our research in this field deals with the synthesis and full characterization of biradicaloids of the type [E(m-NTer)]₂ (E = element of group 15, Scheme 1 species 2).^[1-3] The reactivity of these biradicaloids 2 was employed to activate small molecules bearing single, double and triple bonds. Addition of chalcogens (O₂, S₈, Se_x and Te_x) led to the formation of dichalcogen bridged E₂N₂ heterocycles. In formal [2+2] addition reactions small unsaturated compounds such as ethylene, acetylene, acetone, acetonitrile, tolane, diphenylcarbodiimide, and bis(trimethylsilyl)¬sulfurdiimide are re-adily added to the E2N2 heterocycle of the biradicaloid 2 yielding novel heteroatom cage compounds. The reaction with CO and isonitriles led to the formation of new cyclic 5-membered heterocycles featuring also biradical character. Oxidation with silver salts gave stable cyclic radical cations (3⁺).

Group 15 open shell singlet biradicaloids such as our [P(µ-NTer)]₂ (2) can be utilized in the activation of stable small molecules. For example, fast reactions with H₂, CO₂, and NH₃ were observed. Dihydrogen easily adds to [P(µ-NTer)]₂ yielding [HP(µ-NTer)]₂ at ambient conditions while the reversible release of molecular hydrogen was found at slightly elevated temperatures (T > 60°C).

[1] A. Hinz, R. Kuzora, A. Schulz, A. Villinger, Chem. Eur. J. 2014, 20, 14659 – 16673.

[2] A. Hinz, A. Schulz, A. Villinger, Angew. Chem. Int. Ed. 2015, 54, 668 – 672.

[3] A. Hinz, A. Schulz, A. Villinger, Angew. Chem. Int. Ed.2015, 54, 2776 – 2779.