1,3-Dipolar cyclisation reactions of nitriles with sterically encumbered cyclic triphosphanes: synthesis and electronic structure of phosphorus-rich heterocycles with tunable colour.

We describe the synthesis, solid state and electronic structures of a series of tunable five-membered cationic and charge-neutral inorganic heterocycles featuring a PCN core. 1-Aza-2,3,4-triphospholenium cations [(PR)N(H)CR'], [1] (R' = Me, Ph, 4-MeOCH, 4-CFCH) were formed as triflate salts by the formal [3 + 2]-cyclisation reactions of strained cyclic triphosphanes (PR) (R = Bu, 2,4,6-MeCH (Mes), 2,6- PrCH (Dipp), 2,4,6- PrCH (Tipp)) with nitriles R'CN in the presence of triflic acid. The corresponding neutral free bases (PR)NCR' (2) were readily obtained by subsequent deprotonation with NEt. The PCN cores in 2 show an envelope conformation typical for cyclopentenes and present as yellow to orange compounds in the solid state as well as in solution depending on both substituents R and R' in (PR)NCR'. The PCN cores in [1] show a significant deviation from planarity with increasing steric bulk of the R groups at phosphorus, which results in a decrease in the HOMO-LUMO gap and distinct low-energy UV-Visible absorption bands. This allows access to colours spanning red, blue, indigo, and magenta. TD-DFT calculations provide valuable insight into this phenomenon and indicate an intramolecular charge-transfer from the HOMO located on the P framework to the N[double bond, length as m-dash]C-R'-based LUMO in the cationic species. The cations [1] represent rare examples of phosphorus-rich heterocycles with tunable colour, which can be incorporated into polymers by post-polymerization modification to afford coloured polymers, which demonstrate utility as both proton and ammonia sensors.