Isolation of Neutral, Mono-, and Dicationic B2 P2 Rings by Diphosphorus Addition to a Boron-Boron Triple Bond.

The NHC-stabilised diboryne (B2 (SIDep)2 ; SIDep=1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene) undergoes a high-yielding P-P bond activation with tetraethyldiphosphine at room temperature to form a B2 P2 heterocycle via a diphosphoryldiborene by 1,2-diphosphination. The heterocycle can be oxidised to a radical cation and a dication, respectively, depending on the oxidant used and its counterion. Starting from the planar, neutral 1,3-bis(alkylidene)-1,3-diborata-2,4-diphosphoniocyclobutane, each oxidation step leads to decreased B-B distances and loss of planarity by cationisation. X-ray analyses in conjunction with DFT and CASSCF/NEVPT2 calculations reveal closed-shell singlet, butterfly-shaped structures for the NHC-stabilised dicationic B2 P2 rings, with their diradicaloid, planar-ring isomers lying close in energy.